One thing I find useful is to make a '5 minute' deal with myself. If I don't feel like doing something I should, I do it for 5 minutes. When 5 minutes is up, I can stop if I still want to. Once you've exercised for 5 minutes, you won't stop until you're done.
It's also helpful to keep some motivational messages in front of you.
For example:
Let the mind control the body, not the body control the mind. --Jimmy Johnson
Find some here:
Quotation Collection Quotes on exercise
and here:
Quotation Collection Quotes on health