Locate all of the electrical outlets to see what you're working with. Ideally, you want to be relatively close together and position yourselves as you would on stage. If the position of the outlets isn't conducive to this, opt for power strips and extension cords. Also, keep in mind that you don't want to overload any circuits; use the extension cords to evenly distribute how many outlets you are using.
Compartmentalize your space. To get the most out of the actual performance area, keep all other items in a separate area of the space. No one needs to get hurt tripping over merchandise bins, soda/beer bottles and spare snare drums. Many practice spaces will have a loft; utilize this area if you have one.
Position the drum set first. It should be towards the back wall, as close to the middle as possible. It's less portable than the guitars, microphone stands and amplifiers; moving it around repeatedly would be tiring and counterproductive.
Place the rest of the amps and band members around the drums, with the speakers facing out at a 45 degree angle from the wall you are against. On stage, the speakers will probably be facing directly at the audience, but in the space, you don't have monitors, so the singer and the drummer will need to be able to hear themselves. The more of a semi circle you have, the better.