First, a note about the latest passport regulations: While Mexico still accepts any legitimate ID or proof of citizenship for entry, the United States has required citizens traveling by air to carry a passport since January 2007. The requirement will be extended to land and sea travelers on June 1, 2009. So don't fight it: Get a passport if you don't already have one. The new, credit card-sized Passport Card, available since July, is less expensive ($45 as opposed to $100), more convenient and usually quicker, but it is good only for land and sea travel.

ENTERING MEXICO

Hang on to your Tourist Card

Mexico requires a Tourist Card, also known as an FM-T, for all travelers going beyond the border zone (the area between 20 and 30 kilometers/12.5-18.5 miles of the U.S. border, depending on the location). U.S. citizens traveling within the border zone for 72 hours or less don't need a card. You can get them from Mexican consulates, border crossing points, Mexican tourism offices, airports and most airlines serving Mexico. The fee (about $23) is usually included in the cost of airfare or cruise packages; land travelers will pay and fill out the card at the immigration checkpoint. Keep the card in a safe place; you will have to surrender it when you leave the country or pay $40 to replace it.

Travelers under 18

A person under 18 traveling with only one parent must carry a notarized letter of consent from the other parent (or a decree of sole custody for the accompanying parent or a death certificate for the other parent). Children traveling alone or with other adults need notarized consent from both parents, or from a single parent along with documentation that he or she is the only custodial parent. The letter must be the original, not a copy, and should include travel dates, destinations, airlines and a brief summary of travel plans. Even if you're living on your own and traveling with your college buddies, if you haven't reached your 18th birthday, this applies to you.

Choose your crossings

The U.S.-Mexico border has 25 ports of entry, some with multiple crossings. The busiest of all is the San Ysidro crossing south of San Diego, where you can wait for 1.5 hours or more to get through. El Paso, Laredo and Brownsville, Texas, are the other busiest points, while Columbus, N.M., and Presidio, Texas, often have no wait at all. Travelers can monitor wait times on the Customs and Border Protection's Border Wait Times Web site. You can cut the wait by avoiding morning and evening rush hours and weekends and finding alternative crossing stations. For example, Calexico's East crossing usually has less of a wait than the downtown station, and Otay Mesa, which gets less backed up than San Ysidro, is about five miles east. Andrade, in California but only eight miles west of Yuma, Ariz., is less crowded than San Luis, just south of Yuma.

Determine whether you need to declare any items with Mexican Customs, and have your documents organized and ready; see Mexico's National Institute of Migration site for detailed requirements. Get in the proper lanes — "Nothing to declare" (Nada que declarar) or "Declaration lane" (Carril para declarar). (RV drivers should pull into the declaration lane, because customs usually wants to inspect them.) Avoid lanes reserved for commercial vehicles.

Know what you can bring in

To state the obvious, don't carry anything illegal, particularly drugs, firearms or ammunition. Take only as much medication as you will use during your trip, and carry your prescription with you. Don't bring multiple computers, cameras, cell phones or any other toys it looks like you might try to sell. If you are bringing donations or gifts to Mexico, go to the Declaration Lane and be prepared to document the origin and value of the goods, and be aware that you may not bring used clothing or textiles for charitable donations. If you bring more than the amount allowed, you must pay any assessed duty or pay a steep fine and have the goods seized as contraband — along with your car. You won't pay duty on personal items — within limits. In general, bring only what you will need while traveling. In addition to clothing and toiletries, you are allowed two cameras or video recorders, one laptop computer, one PDA, one CD player, two cell phones, 12 rolls of unused film or videocassettes and five memory cards. Visitors over 18 may bring 20 packs of cigarettes, 25 cigars and up to 3 liters of liquor and wine. For the exact number of personal items allowed, and for details on required vehicle permits, see the Mexican Customs Web site, Aduana Mexico. (Click on the "English" tab at the top of the page for English instructions.)

Passing inspection

After immigration, you will go through customs; have your Customs Declaration Form filled out and handy. Mexico has adopted a red light-green light system for travelers who have marked "Nothing to declare" on their forms. At the airport, you will be asked to push a button; if you're driving, a traffic light will turn red or green when you pull up to the checkpoint. If you get a green light, which you usually will, sail on through. If it turns red, you are one of the lucky few selected at random for a secondary inspection. If you have no guns, drugs or other contraband, you will just have to answer a few questions and you'll be on your way. Land travelers should verify at the border that all their belongings are legal to bring into Mexico, because they will face a second immigration and inspection south of the border. Prohibited items, even if they passed the border inspection, may be seized and you could be prosecuted.

HOME AGAIN

Patience is a virtue

Coming back into the United States can be more complicated than leaving it, and northward border crossings take correspondingly longer. U.S. Customs requires you to declare everything you acquired in Mexico, whether you owe duty on it or not. It's a good idea to keep receipts. And keep in mind that Customs officers are allowed to inspect you, your luggage and your vehicle at will, without a warrant. The good news is that if you were traveling for leisure, you most often will not be searched at all; lone males and groups of college students returning in springtime are most likely to be searched.

What you can bring home

U.S. Customs regulations are more comprehensive, and convoluted, than Mexico's. Generally, you're allowed to carry one liter of alcohol for personal use before you have to pay duty; remember that even though you can buy alcohol in Mexico at age 18, you must be 21 to carry it across the border. Uncooked meat and poultry products, and pork of any kind, are prohibited, though you can bring fish you caught in Mexico. Most fruits and vegetables require an import permit, and dairy products usually are not allowed. To be safe, stick to baked goods, chocolate, candy. Many foreign-made medications are not FDA-approved and will not be allowed into the United States, nor will any product made in Cuba. Animal products other than normal leather will be confiscated, including sea turtle shells, alligator skin, bird feathers, and even black coral and shells. Taking any sort of pre-Columbian artifact home is considered theft; you will be arrested and prosecuted.

THE FINER POINTS

Play it cool

Looking reasonably clean-cut is good, but if you're decked out in Armani, Rolex and Coach, you might be asked to step inside for a spell. Flashy clothes and jewelry are also an invitation to Mexican authorities to hit you up for a few bucks. Don't stare at border officers, but don't try to avoid them, either. You're aiming for nonchalance.

Be tidy

If you are driving, consider giving your car a thorough cleaning before you head south. A bullet that rolled under your seat during last year's hunting trip or a marijuana seed from the joint that your, ahem, brother tried five years ago will land you in a Mexican jail just as quickly as a gun or a baggie.

Don't lie

Fill out your customs form, truthfully and comprehensively; there is no way to predict the random search, and trying to get something through could land you a steep fine, or worse. When U.S. officials ask you where you've been in Mexico and how long you were there, tell them, even if you get inane questions ("What are you smuggling from Mexico today?" or "What if I told you we're looking for a car just like yours that was reported to have marijuana inside?"). Stick to simple, polite "yes" and "no" answers; if you stumble over your words or contradict yourself, don't be surprised to be told to open your bags.