[what he said]
Organic matter will lighten the soil too. If you can find composted rice hulls or saw dust that could help a lot. Be sure it is composted well, or you will need extra Nitrogen to offset the bacterial N demands. But anything that feeds the soil microbe community will help.
For inorganics, I like gritty material. Washed coarse sand or DG will work well. I would only buy bulk from a landscape supply store though. FORGET BAGS!!!!!!!!!!
For my beds, I like to add a thick layer of deciduous hard wood leaves every fall. This can be chopped into the bed in late winter. If you have a large hardwood near by, start a leaf pile and use it as mulch, or make leaf mold which is possibly the best soil conditioner there is.
Also, if you have a lawn, consider bagging your lawn clippings. Green Grass has a high nitrogen to carbon ratio. This means it composts fast and without creating a N deficiency like woodier material will.
Also, be sure to mulch the walkways with woodchips. This trick works great to keep down weeds and mud, but also functions as a compost bin for the extra space. Roots will not be confined to beds alone, so creating a cheap and fertile soil zone in walkways helps also.