An investment in one of these will save you headaches in the long run.
This way, you can be sure you're actually bleeding all of the air pockets out of the system, instead of constantly having to add coolant. This also creates a high point in the cooling system, allowing the air to come up. You can also rev the engine with this on and help get those annoying air pockets out.
You can try flushing the system out in case there are rust deposits, ie "flakes," to see if that will help, but I would get a funnel in the long run for any future drain and refills.
You sure the thermostat is opening when it should? Did you take it out and put it in a pot of boiling water?