I use blood I accidentally shed on my runes or other sigils. Just yesturday I accidentally cut open my cuticle (hurt as much as it sounds) and saw the drips of blood and immediately used it on a personal seal drawn on a stone I keep on my altar. I believe in "waste not" =)
But if it is for sacrifice, as an offering, a small cut or pinprick should be made on purpose. Fingertips are best because they heal quickly and stop bleeding fast.
In adding personal power, painless alternatives to blood would be semen or menstrual blood. Urine is controversial in use, everyone has different perspectives on when and how it should be used.