I donate every time the bus comes near work (about every 3 months). It's a good way to get over that fear of needles, eventually you'll be watching them do it

. Drinking lots of water helps them find the vein and keeps the blood well hydrated so it flows well. Start drinking more than usual the night before for best results

. I can definately recommend eating something at least an hour before donating, as I donated once feeling hungry and that didn't end up so good. Nothing a quick snack doesn't fix but it's best to avoid that. The actual donation usually takes about 5-10 minutes, but if they're busy (which is usual for a bus) the process can take up to an hour.
Afterwards just take it easy, drink lots still and have a decent feed. It'll probably take a day or two before you can do heavy exercise again, but just keep it in mind, if you start feeling light headed take a break. It's a good excuse to be lazy for a bit.