Avoid missions during heavy rain as Chatuchak's drainage system can easily overflow. As much of the market is given over to selling pets and wild animals, you could end up wading shin-deep through feculent, brackish ooze. Not a good call if you've been scratching your infernal mosquito bites raw—unless, of course, you intend to go shopping for tetanus, septicemia or worse.
Meanwhile, you should be drinking water—and lots of it. The mercury under Chatuchak's corrugated-tin canopy can easily top 40°C. To avoid the worst of the heat, attack early—the market starts to open at 7 a.m., and most stalls are fully operational by 8 a.m. Note also that Chatuchak gets unimaginably busy, so don't operate in an unwieldy brigade. Split into mobile hit squads of twos and threes, and arrange to regroup at the prominent central clock tower (the vital landmark was donated by the market's Chinese Association in honor of the Thai King's 60th birthday in 1987).
Lastly, avoid the temptation to escape with your purchases by taxi or tuk tuk. The air-conditioned Skytrain is only 100 meters away from the northeastern market entrance at Mo Chit station (before the rail system was built, it took an hour to get here from downtown). And as Mo Chit is the northern terminus of the Sukhumvit line, you've got a good chance of a seat—plus you'll also avoid the City of Angels' hellish traffic snarls.