DRINK WATER! At least 1 litre in the 2 hours before. In the waiting room is not too late.
Doesn't matter if it makes you pee, you won't pee it all.
Unless you are donating platelets. Because you will be lying there about two hours....and if you have to pee for the last half-hour of that time, you will be miserable when it is all over. In fact, if you are donating something that takes a while, remember to go potty before you start. There are no potty breaks mid-donation. Probably not as important for the shorter donations....still, you don't want to realize mid-donation that you don't have your astronaut diapers on and that nothing else will save you.
Tell them if you have had trouble donating in the past, or been a tough stick. They will generally give you someone who is more experienced, which should make you more comfortable (and relaxed) which will give you a better experience. And there is no need to scare some poor newbie into thinking that everyone has little tiny collapsing veins and they will never be able to do the job. :>
If it hurts or just doesn't feel right, let them know. They can't do much if they don't know that you are having a problem. Sometimes fixing things can be as easy as moving the needle a little to get it away from the vein walls.
If you have the time and the ability, ask about donations other than whole blood. My husband always donates whole blood (he is O+ which means that is what they want from him). I, on the other hand, am A+ so they would take my blood....but they have found that I have a very high platelet count. I generally do a triple donation in less time than it takes most people to do a single or double. Some of the other donations may take a little longer, but if you can give them the time it may be just what they need. If you are doing something that takes longer, be sure to wear comfy clothes. If I donated whole blood, a business suit would be okay as it doesn't take me long to fill a bag. But for sitting for a long donation, best be wearing something that is comfortable. Since my feet get cold during donations, I have been known to bring my fuzzy slippers and put them on during my donations.
Oh-and you can add me to the group that watches the needle go in....I like to see if they got a good stick and get blood flow. I also don't get dizzy from donation (never have in all my years, anyway), and tend to watch the lines/bags to see how things are going.