Get a 5Ghz Access Point - yes the range is slightly smaller but the channel overlap and other device noise is much less of an issue. Most devices manufactured in the last 5 years support 5Ghz. Use a different SSID for your 5Ghz network and only configure your supported devices to use it instead.

Add a second Access Point. Most AP/Router combos allow you to configure them in AP only mode. Set it up with the same SSID if you like. Then you just connect the LAN side to a LAN port on your other AP and you'll end up with two access points. Put them at extreme ends of eachother or near places you need good signal.

Lower your Access Point power. This is a little counterintuitive, but especially if you have more than one access point, you want to think in terms of 'microcells'. You don't want your laptop clinging on to your far away AP's strong signal, you want it to purposefully roam to the nearer AP.

Boosting the signal as strong as it will go will cause lots of overlap in coverage, and leads to the signal overload situation that we now have in the 2.4Ghz band.

For OSX, I like WifiExplorer (in the app store)