When you're at the Red Cross giving blood, make sure that you are prepared for the questionnaire to be lengthy and to include embarrassing questions! Last time I went, I was able to complete it privately on computer, which saved time, but there are a lot of questions like, "In the last twelve months, have you had sex with a male who's ever had sexual relations with another male?" Or lived in Africa; or if you've lived with a person with hepatitis, and on and on. They ask everyone these questions. It's not personal!
I would also encourage people to plan on taking it easy for the rest of the day. Some people will bounce back very quickly, but other people will find that they're tired after giving blood and will need to rest. I once had a difficult donate (the person had had trouble finding the vein) and almost passed out in a fabric store! Now I drive straight home. It's a nice excuse to have a lazy day and feel really good about it!