The way you set up will make a lot of difference. If you set up like you would for a gig (everything faceing forward) getting much seperation is cloce to impossible. Keep in mind that in the studio, it has to sound good, not look good. First off, spread things out as much as possible, distance is the tool for all at once recording. Second, face amps away from each other and away from the drums, avoid setting them in corners faced out. Third, cloce mic things as much as possible. Less sensitive mics, placed cloce will work better than sensitive mics spread out. Fourth, play at lower volume and with less distortion and have everyone listen through headphones rather than to the live sound in the room. Fifth, isolate the vocals if you can, a seperate room is ideal, if you can't use another room use a cloce proxcimity mic for vocals.
Set the drums up first then set amps up around them (faceing away from the drums) with some sort of baffel between them. DI the bass and vocals. When recording "all at once" you have to create space between the different instruments. You have to create "sonic space" for each instrument or everything will bleed together. You will never completely get rid of all the bleedover but it can be reduced to a workable level.
I hope some of this helps. Most of my ideas come from trial and error, these are just some that work for me, hope they will for you.
