+1. Make sure you have good ergonomics. There's a lot of threads on this. Bends in your wrist will put extra strain on them and your forearms. Don't try to be "cool" slinging the bass low.
On the fretting hand keep your wrist straight and don't grip the neck like a bat - just lightly use your thumb on the back to stabilize. You may have to hike up your strap to get a good position. Don't fret any harder than you need to, try to move your fingers as little as needed, and don't take your pinky behind the neck or other weird stuff.
On the plucking hand again, the bass body should be low enough to keep your wrist mostly straight, and if you wrest on a finger rest or pickup do it just enough to stabilize your hand, don't put any pressure on it. Try to minimize your strokes and use a light touch.
Understand that many, many people play without good position, even rich and famous bass players, so use your own judgement
Also, these things will help you play faster and with better tone.I also think there's benefit to the stretches other people mentioned. I like to do some forearm weight lifting excercises at the gym too - take dumbbells or a cable machine and do wrist curls with palms facing both up and down, as well as wrist rolls.
