I'll be there at the end of January to begin another tour of Thailand for a month. I recommend staying in the Sukhumvit area, near the BTS line. Hotels are very reasonable and it's a quick, cheap train ride to the major shopping areas. MBK mall is a must to check out, you can find knock-off clothing that's near indistinguishable from the brand names for super cheap. And if you're staying in the area, you should make a stop at Nana Plaza for a beer at night to watch the madness. It's the reddest of red light districts
If you're on a really tight budget, you can find really cheap accommodations around Ko San Road, which is the backpackers hangout. Tons of bars and a really fun place to spend a night drinking, though the trinket peddlers can get a little annoying after awhile. One negative is that it's no where near the mass transit lines, so you'll be cabbing it to go anywhere
And though Drak is right about the cabs not wanting to turn their meters off and charge you more, it's not really worth it to take a hard line on meters only sometimes. Last time I was partying at Ko San I could not find a single cab with their meter on and every cabbie was quoting me double what a regular fare would cost. But then I stepped back and realized that I was arguing with them about a difference of $3 (100 baht vs 200 baht) and though what the hell am I doing? Even getting "ripped off" by a cab in Bangkok will still be 10x cheaper than paying a regular fare anywhere in north america