You only have to clear customs at Heathrow if your bags are not checked through to your final destination. I've never had to do it (but then I always use Star Alliance airlines - United and partners). If you've booked on the SAME ticket at the same time, then you probably won't have to clear customs at Heathrow.
You'll know because all of your flights will be on the same ticket, even if you have different airlines listed.
If you've booked two separate tickets that are not connected to each other to save money, then I hope you allowed plenty of time! You'll have to go through Passport Control, clear Customs, then re-check your bags and go through passport control and security again. That's because when you check your bags in the States, they won't be able to check them all the way through.
But this is actually quite rare for flights coming from the US, and booked on travel sites like Expedia, etc.