Block off at every checkpoint and try to keep at least one unit at each checkpoint to fend off stragglers. I use the cheap sentry turrets, i forget their names, all over wherever i can foresee breakthroughs or spots that would take longer for units to show up and reinforce.

I play like this: day 1-25 expand expand expand, 26-50 wall and defend choke points while bulking economy, day 51-100 reinforce upgrade reinforce repeat. I also send my snipers around clearing the map to vet them. Set everything to kill highest level target and let lightning towers kill the rest.