Here's what I do (but I'm eager to see how others do it better):
Take an L bracket and hose clamp or take a short piece of flat metal and tack weld it on the end of your tube-- the last part that would go into the bender if you went that far. Check it with a level before bend #1 but while you've put some tension on the bending lever.
After you complete bend #1, check to see where the bubble falls again.-- usually I tweek the bracket again because I know the bender "settled" from me pushing/twisting on it while bending, but I take note of the difference.
When you position the tube for bend #2, put some tension on it again (support however you need with a chair back or step ladder or whatever) while you maneuver around and set the level mark again to how it was when you were finished with bend #1 or possibly a smidge toward where it was before you did bend #1 but not all the way to the pre-bend 1 location.
Go ahead and start bend 2 (where it is supposedly even with the first bend) and do a degree or two so it sorta "sticks" without the tube sagging. Then go back and check the level. You can often times adjust if you need to a tiny bit if you need to.
I do get better results when I have a helper holding the other end of the tube, but there have been times when the "helper" is lifting too much and throwing off the level mark.