When to go: the area has a spring-like climate all year round, so just about anytime is a good time to go. July is technically part of the rainy season; however, it did not rain very much while I was there. I found the weather very pleasant and could see why it's such a popular place to retire.
How to pick a place to stay: I, of course, recommend Casa de las Flores wholeheartedly. But don't just rely on my advice! Check out reviews on TripAdvisor to see what hotel or B&B would most appeal to you. What will really enhance your stay is if you can get personal attention from your host or the concierge, so they can help you plan trips and arrange for a driver or whatever else you need. Ask what services they offer! A driver can often double as an interpreter and for a relatively reasonable price take you wherever you want to go. Luis charged $20/hour with a minimum of 4 hours.
What to wear: as Stan at Casa de las Flores says, the biggest danger in Tlaquepaque is the uneven pavement and streets. So pack some sturdy walking shoes. I bought a pair of Keen Carmel sandals for this trip, and they worked fabulously. I wore short sleeved shirts and jeans for most of the trip, and I felt both comfortable and appropriately dressed. You can dress casually (neatly, though) like you would at home. Pack a sweater or light jacket for the evenings, though, when it can get cool. To not look like a target, don't wear flashy jewelry or an expensive handbag. I swear by using a PacSafe bag for peace of mind.
Language: basic knowledge of some common Spanish words and phrases ("Buenos dias," "Gracias," "Cuanto cuesta?" "Donde esta el bano?") will go far. In high-end restaurants and shops, people will speak English; however, be prepared to communicate in Spanish - at least a little bit - with vendors in the markets. Knowing numbers in Spanish is often necessary for shopping, but you can also gesture for the seller to write down the price on a piece of paper. If you have no Spanish whatsoever, don't worry! If you use a hotel like Casa de las Flores, for a reasonable fee you can arrange to have an interpreter.
Food safety: everyone knows you're not supposed to drink the water. You also need to watch out for ice and any fruits or vegetables that have not been cooked or peeled. But that doesn't mean you can't have fun and eat well! Here are my ground rules: I took chances (had beverages with ice or tried the pico de gallo) only at the best restaurants in town, where they catered to international tourists and well-heeled Mexicans. Prices there are still reasonable by American standards. Street/market food is generally safe to eat if is steaming hot, right off the stove.
These are some initial tips, especially geared towards those of you who have never traveled to Latin America. Stay tuned for some more!