some simple stretches when you come off should do the trick

kneel down with your feet flat and put calves n thighs together
foot on saddle and stretch the leg pulling the toes back towards you
lean forward keeping on leg straight that you are putting pressure on

I had a knee injury last year and I swear by the balance board that I bought whilst doing rehab with the physio, gets the balance muscles around your knees working, also working on your balance is good for you in other sports for me inline and ice skating as well as skiboarding.

I have now worked up to doing a series of 1 leg squats on the balance board...

for a ride to work you shouldnt need a warm up as many gyms use a bike as part of the initial stage or a warm up routine..... but try it and see if your body appreciates it....