Congratulations MIlly on being a blood donor - such an integral part of modern medical treatment, so reliant on the kindness of volunteers from within the community - and only around 3-4% of the population are registered blood donors. You can justifiably feel very proud of yourself. :)
You have probably received all the answers you need anyway, but just to add my take;
1. Does blood have a "sell-by date" - Yes, it does.The blood you donate is collected into plastic bags containing an anticoagulant / preservative.
The plasma from your donation is then removed, using a process of centrifugation, and that plasma is frozen and stored, ready for processing, to produce either a plasma substitute, or for further fractionation, to provide specialist plasma protein products. Actually, it is slightly more complicated than that nowadays, but it serves as a good basic model of what happens.
The remaining red cells from your donation, once screened, grouped, and processed, then have a life-span of approximately 35 days upon reaching the hospitals/clinics. 99.9% of all donated red cells are used up in transfusing needy patients during that period.
In the UK., the frequencies of O+ and A+ are very similar, O+ being slightly more common. (37% vs 35%)
You can donate for red cells every 16 weeks or so, if memory serves. Finally, frizzer has given you a very good link, I can offer one more from Wiki, which is a very good overview of blood donation.
http://en.wikipedia.org/wiki/Blood_donation
Its a fantastic gift.