Here's how I do it:
- Get a length of waxed dental floss
- Thread it through the spring
- Compress the spring using tweezers
- Walk the spring in place
- Slowly pull out the dental floss and the spring should lock in.
- Repeat for the other springs.
When I disassemble trucks, when it comes time to pull the side frames, I put the truck into a plastic bag so when the springs go flying they're in the bag.
Hope this helps.