Trying to fly open jaw Seattle-Warsaw then London-Seattle

Don't wait until the month before. The general subject of how to get the best airfares has been frequently discussed here, retrieve these discussions, including people's favorite websites, by using the search box in the upper right corner using appropriate search words. Specifically, I don't have any inside knowledge of the pricing policies of airlines, but my guess is they probably base their price more on the date you want to fly, at least that has something to do with the price, and if that is correct, then you wanting to return in August makes it more difficult to get "winter like prices." Maybe others who know more about strategies for buying airfares will be able to suggest something. Probably you have to start researching airfares and at a certain point take the best deal you can find.

I am not expecting to get a cheap flight back to the states, but I thought there is still a chance of getting a bargain for leaving in April. Currently I am unable to find anything less then $700 to Warsaw in April, but as far as I know its only that expensive because most flights arent planned more then 4 months in advance.

Have a good look at flying with Lot the Polish Airline. They are members of the Star Alliance which also includes UA. So it is possible for them to be able to fly you into WAW on a Lot flight and out of LHR on a Lot codeshare with UA.

The best of luck. There will always be a bargain hidden somewhere but I've got a feeling prices are not going to be reduced by much in 2008.

I'm not sure where the four months comes in. All IATA scheduled airlines have to post flights 360 days in advance. More and more passengers are booking a long way ahead, often six to nine months.