I've noticed improvements by doing running intervals, where I'll jog easily for about 10 minutes to warm up, then sprint all-out uphill for about 20 seconds, then jog easily for about 2 1/2 to 3 minutes to recover, then repeat ad-nauseaum. I try to do this for about 30 to 45 minutes total, once a week. I basically accomplish this by running around a block several times with a steep uphill on one side of it. This seems to replicate the exhaustion I feel during a race. In addition to that, I may do an easy 30 to 45 minute run during the week just to keep the joints and muscles supple. Other than that, I just hit the rollers a couple times a week at varying intensities, and do the Wednesday night training races they have here in the Twin Cities.
For me, the purpose of training is to make the races fun, instead of grueling death marches. I only train enough to not embarass myself out there on the course. If I place well, that's an added bonus.