Well, you'll definitely want to make sure the router has the least amount of obstacles between it and your computer that's trying to obtain signal.
You'll also want to limit factors that cause interference like 2.4Ghz phones, especially if they're DSS (Digital Spread Spectrum), microwaves, and probably numerous other devices.
That's one thing to check. I also noticed that in my case -- I don't have anything wireless N yet so, I told my router to use Wireless G and that's helped significantly.
Also, some of the channels on the spectrum are non-overlapping so you may want to manually assign one and see if results are better. Or, this DIR615 supports auto finding a channel with 'least' amount of interference so you might see if that is enabled.
Those are just a few things you can try.