Having only been there twice I really cant call myself an expert on the place, but I do have some advice for you on this.
1. Dont plan anything, dont book anything in advance! The fact is that these days Bali is so easy to get to other places, booking it is a waste of time and money. Get there, go into an internet cafe, check the forecasts, then make the decision.
2. Dont go to G-land expecting to get all time Speedies, it rarely ever happens and if it does, half the surfing population in Bali will join you. Last time I went there Speedies was almost perfect and there were close to 100 people out, with another 50 at launching pads.
3. If the winds are wrong, the whole place is going to be bad. Unfortunately, there isnt much to do in G-land if the surf isnt cooperating. If you stay at Bobbys camp they will put on movies for you all day and you can lay on a lounge in air con.
4. If its gonna be big, be prepared! If you havent been there before, chances are you are not going to try to take on 10-12ft moneytrees on your first day. Be realistic in your expectations of what you are willing to take on. Its not the ideal dry hair paddle out most people imagine. Getting in and out can be very tricky. Especially getting out if you miss the keyhole usually means you have at least another 20 minutes of straight non-stop paddling to get to where you want to be.
5. There are other places to surf, its just they arent all that great. Chickens, 20/20s, tiger trails, etc. are all fun waves on their day just dont expect them to be perfect square slabbing barrels
I should take a more upbeat tone with the rest of this post...
6. If its flat in Bali, its 2-3ft at G-land. If you are sitting at Ulus watching 200+ people fighting over 1-2 ft racetracks, I would say go to G-land immediately. Not a lot of people think of going to G-land when its small, when in my opinion its the best time to go. I surfed Kongs for three days straight on my last trip, 8hrs a day and had an absolute ball, never had to hassle during that time and got more than my fill of waves. Meanwhile Ulus was small and crowded and there were few alternatives in Bali other than the bingtang mayhem that usually ensues.
7. The help at the camps are very poor people, but they have plenty of surf wax, boardies, and sh!t t-shirts to keep them covered. Bring them something they cant easily get their hands on. The last time I went I took them guitar strings, picks and a tuner, the looks on their faces were priceless. And they were more than grateful, they took very good care of me.
Like I said, I am not an expert, but over the two trips I have taken there I have realised a few things about what works and what doesnt in terms of going there.