Was just there early to mid December, posted a detailed trip report on two other forums catering to international exploits. Shouldn't be too hard to find if you're interested.
1 - In a nutshell, if you know your way around the usual places (NEP, Cowboy, seems you do), just be prepared that any go-go excluding Baccara and Rainbow 4 is subject to change nightly. When you're there for a week +, the differences over the course of it can be marked. Some nights a bar is pumping, stocked with quality AND quantity of girls. The very next night it could be dead and they'll try to rort you for Lady Drinks just to get some money through the place. This happened to me at Mandarin (NEP), went there 4 nights all up, resulting in 3 different bar fine's but one REALLY bad night where they rorted me. So, don't rely on any bars in particular, treat every night as a seperate mission and find what you're looking for on that night as quickly as possible. As stated, Baccara and Rainbow 4 are the exceptions. They are ALWAYS loaded with honey's, but beware that they're both VERY Japanese-oriented and it's not always a given that a farang will be able to score his lady of choice.
2 - Go to Nataree. In my opinion, best "massage" joint in BKK. The rooms are old and in disrepair, but assuming you're not there for the decor, you'll be hard-pressed to find a bigger collection of fresh young sideline-stunners starting at 2,300 baht for up to 90mins. The papasan's are mostly ace, have a good chat in advance if you've got any specific requests, as not ALL girls will give you 2 shots and you could be out of the room in an hour if you blow early with the wrong one. Most are incredibly willing though, so should be agreeable to reasonable requests for a small tip. Others are freaks and will just out-fuck you for fun. :-) Also, have a spare 20 baht note handy, it's standard to leave a tip for the maid after you're done and I don't think it's right for the girls to pay it out of their own pocket.
3 - Avoid the "coyote" bars owned by the "Iranian" or "Arab" as he's known, on Soi Cowboy. They're the places that you're certain to get bent over for LD's, and often find your bill has been padded just to add insult to injury. Deja Vu is the worst, but I'll still stop in there for a "McPiss" given that they have a special window allowing you to view the stage from the comfort of your urinal. :-) Midnite is the ONLY one of his bars worth spending time investigating a bar fine imo.
4 - Take your own rubbers, and lots of them.
5 - At NEP, ALWAYS take in a show at Angelwitch, and ALWAYS check out the jacuzzi at Billboard. If you like fresh "spinner" types, you'll thank me for that last tip, it's a beautiful thing seeing between 3 & 6 nubile little minx's gyrating around in a giant tub wearing nothing but strategically placed suds.
Enjoy!!!