Improving core strength
This has had a dramatic effect on being to hold a good position on the bike:
Begin by standing on a comfortable surface, where you have plenty of room at each side. With a 2-kg potato bag in each hand, extend your arms straight out from your sides and hold them there as long as you can.
Try to reach a full minute, and then relax. Each day you'll find that you can hold this position for just a bit longer.
After a couple of weeks, move up to 5-kg potato bags.
Then try 10-kg potato bags and then eventually try to get to where you can lift a 20-kg potato bags in each hand and hold your arms straight for more than a full minute. (I'm at this level.)
After you feel confident at that level, put 1 medium-sized potato in each of the bags.