Drink a lot of water since the previous day: the general recommendation to keep an optimal level of hydration is taking 0.25 gallons per 55 pounds of body weight (1L per 25Kg), so if you want to be extra hydrated and get the blood a little more fluid try to at least drink that quantity the day before + a glass of water 20 mins before the collection.

Before taking the sample, leave your hands under warm water for a while (1 min is ok).

Clap your hands a few times really hard to increase blood flow in your hands just before pricking it. Opening and closing your hands a few times also helps to stimulate the blood flow.

Stand up while taking the sample and put your hand at your stomach height, so it is below your heart, this makes the blood flow much higher.

Massage your finger from the palm or even your wrist, until the top of the finger while the blood is flowing (don't stop until you are done collecting the sample). Just avoid the puncture so you don't get blood on you.