also, just try and think say 3 months into the future... by Xmas, if you've done nothing, you'll look back and think" I could have done 3 months of solid training there, what a waste"... so I always have a target for 3-4 months ahead...
I currently have 3 main ones, the Worksop 1/2 Marathon (sub 01:30) on 30th Oct, the Helena Tipping 10 (38:25) on 4th Dec and the Helsby Four Villages Half Marathon (01:22:50) in the ned of January.
When I feel crap, I just think, if I dont' train I won't do very well, and usually that's enough for me :-)