I have one small rolling vein in my left arm that is the only way techs can draw blood. I have also been through 3 rounds of IVF and four FETs. Here is what I have learned over the last 5 years.
1. Drink water the night before, and chug as soon as you wake up in the morning. Yes you may have to pee at night, but it makes a huge difference.
2. Exercise your arms (strength training). This might sound like ridiculous advice for a mere blood draw, but if you've been through as many as I have, you'll try everything. I started doing small dumbbell weights with my Peloton workouts.
3. Right before your blood draw, do a few standing pushups in the bathroom, leaning on the sink/counter.
4. As you walk to the blood draw, swing your arms around in big circles. You *will* look like a crazy person, but the centrifugal force helps.
5. As you sit in the chair, gently press and massage your vein at the injection site.
6. Ask for a heat pad if you are cold.
This may sound extreme, but I am an extreme case. My first IVF, the techs had so much trouble with me that they were always messing up that vein and having to try to draw out of my hands or wrist, which was extremely painful and left me black and blue. Then I would go back the next day and they would jab those bruised locations again before (a couple times) giving up. Just staying hydrated wasn't enough for me. When I can manage to do most of these, they usually get my vein on the first try.
Additional info:
https://hemaware.org/mind-body/tips-and-tricks-accessing-problem-veins
https://www.ladyoflyme.com/blog/tips-for-a-good-blood-draw-small-and-difficult-veins