It is a good idea to start checking out the various hotels online so you can get a good idea of what the hotel looks like and what they have to offer. In choosing your hotel you should consider the safety, location and price of the place and whether the place is guest-friendly. Always try to book a double with hotels outside of the Nana area, in case they want to charge a guest fee. I am only going to recommend hotels which I know do not charge guest fees. It is only a good idea to book your hotel ahead of time if you are going during high season or during Songkran or Loy Kratong holidays. I would avoid going to Bangkok during December or during Songkran for your first visit.
I recommend the following hotels:

DYNASTY INN: (Soi 4)
By far the best bang for your buck in BKK. I stayed there for three nights for the first time last week and was very impressed with the new rooms. I had zero problems with bringing girls back, even two at a time. The only drawback is that the girls assume you only want short time as the hotel is so damn close to Nana. I paid 1,100 for the rooms in the new wing but also paid 950 baht for the rooms in the older wing and found the rooms to be slightly bigger. You can walk to Nana Plaza or Angel's Disco and the Sky Train is a short distance away. Service at the fron desk is excellent. I would not recommend the food here or the internet spot across the street...both are overpriced and the food at the Dynasty is not so great. They have a travel agency and traditional massage in the facility and a safety deposit box for your belongings. The air conditioner is superb. Laundry service is available at a reasonable cost and is done promptly. Good idea to have the maids iron your clothes for you as your good clothes will be thrashed from the flight. Tip her well and she will do it on the side.

OMNI SUITES: (Soi 4)
Located on Soi 4 off of Sukhumvit. I will probably only stay here from now on.
Cost 1800 baht per night for 1 bedroom condo.
Young men in mock military outfits salute you and your date at the door. Lobby contains small store, internet cafe, beauty salon and travel agent. Need security card to access elevator (prevents bar girls from attempting to surprise you) No need to pay guest charges as it is not a hotel yet it cotains all amenities of hotel except full fledge room service. Room service exists and is cheap but limited. Condo contains small kitchen with microwave, stove and fridge. Living room has several chairs and a large couch with t.v. Bathroom is huge and modern. Walk in closet with in-room safe. Bedroom has a couch and a large queen/king size bed with dresser and t.v. This place could confortably hold a large party and costs a little more than other hotels in the area. Excellent view of the skyline of Bangkok. It's about a 3-5 minute walk to Nana Plaza. I chose the one bedroom suite. I checked out the Grand President and Omni has much nicer facilities. The rooms are beautiful. Your chicks will be impressed. Nice view too. I would not book this place online as the prices for walk-up are always cheaper and negotiable, especially during low season.

NANA PLAZA HOTEL: (Soi 4)
Convenienice here cannot be matched. The place has a disco downstairs and the lobby is full of freelancers. There is also an excellent cafe and good pool. The downside is that many of the rooms are shabby and old from wear and tear. I would not recommend staying at any room below the Junior Suites at 1,250 baht a night or less. The standard rooms are awful. A great place if you like short time girls and dont mind a little older hotel.

GRAND PRESIDENT: (Soi 11)
If you want to pay a little more(2,300 for Royal Suite), this place is excellent. I stayed there one time and negotiated 1,800 for the Royal Suite as it was during low season. They give you cheaper rates if you pay cash and stay there longer. The amenities are nearly as good as Omni and the location excellent.

BOSS TOWER: (RAMA 4)
This place is good if you are staying for an extended period of time and want some privacy. Located about 2 km from the sky train, this place has nice rooms with a great view of the city for 850 baht a night. It is an apartment, so no guest policy problems. The receptionists are hot. The pool is beautiful and has a nice view of the city. The drawback is that the place is not in the thick of things so you will need a taxi to get to sky train. The traffic on this street can be horrible at times.