Can you tell me some donating blood tips for more visible veins and faster donation?

Hand exercise: Get a soft rubber ball and exercise your hand and forearm by squeezing it many times each day- over the course of weeks your muscles will get stronger and in the process your blood veins will enlarge as well. Short term trick is to warm the arm with warm compresses to increase blood flow and dilate veins

Pre-hydrate: Go in well hydrated (don't overdo it so you don't "float" during last part of donation!). Only quick impact on veins & visibility. Longer term, can increase arm exercises or lose weight (if overweight) to improve visibility.