In reply to Removed User:
> One of the main reasons for adopting double ropes was to reduce drag.
Nope, redundance and option of full lenght rappels were the first 2 reasons... only third was the option of reducing drag.
And the drag is only really true on wondering routes that go left, right and back again with few gear placements.
As for the OP and his question...
Single ropes can help (easier to plan the rope line, so that there isn't anything for the rope to snag, get caught and generally have a nice straight line of on with minimal twists)... But the key points have already been mentioned above.
1. Aim for a straight rope line (this means that you might not be able to place gear when you wan't rather you place the gear to direct the rope).
2. If there are potential snag-points, use nuts etc. to block them. Quite often these are cracks in a roof the rope goes near... so place a strategic nut at the lip, so that the rope can't get in the crack.
3. Extend the gear enough to full fill criteria one.
4. If the route is doing too big turns (e.g. come out of bloody tunnel and do a right angle turn), build a belay close by. Remember, the guidebook describes where the route goes... and where the 'marked' belays are, depend greatly on numerous things... in the alps, pitches tend to be 25m or less (because that's the ropes they had). Now it might be smart to link pitches (if line is direct)... same goes the other way, too twisty and/or running out of gear, build a belay somewhere earlier.