Personally, I am extremely 'time crunched'.

I love the idea of following 'The Time Crunched Cyclist' as mentioned by others, but I have trouble finding 3-4 hrs a week as it is. Having a very young family doesn't help. So, I've been following http://www.youtube.com/mtbstrengthcoach & doing about 2 x 20min sessions a week in the garage when my kids are asleep. I also swim twice a week at work for another 20 min.

Otherwise, I just try to ride as much as I can. All this wet weather doesn't help - but garage workouts are better than nothing.