My trick is that I always just buy the base economy ticket that can still be upgraded. Then I check for upgrades every couple days. What I found is that prices to move classes will vary wildly as they try to sell all the seats. It also makes it easier to upgrade a single leg - an overnight flight is more important to be lay flat than a daytime flight for example.

The other part of my equation is that I put a 'dollar figure x flight time' that I'm willing to pay to be more comfortable. If I see a price that hits my threshold I upgrade, otherwise not.