My front deraileur needs adjustment after every ride I have a Cannondale Women's Adventure 1 that I got this spring and have been riding on local (paved) trails every weekend. I started off riding about 6 miles a weekend and recently have progressed to 20-25 miles in a day.

About halfway through a ride, my front derailleur seems to need adjusting, I have to wiggle the shift knob to get the chain to change gears. For example, if I'm in 1 and go to 2, I have to pull the knob slightly ahead of 2 (between 2 and 3) to get it to change. To go from 2 to 1, I have to adjust the back gears a few times, or sometimes backpedalling will get it to change (but that's hard to do on a hill).

I keep taking it to my LBS (where I bought the bike) and they adjust it so it works fine in their parking lot, but again after riding for a few miles it's messed up again. 

I don't know if I'm doing something wrong in technique or if there's something wrong with some piece of the bike, nor do I know enough about what's wrong to find anything that's helpful. It can be adjusted to shift correctly, but that adjustment doesn't stay for very long.  What should I be looking for? Or, could I be somehow doing something when riding to cause this problem?

Some specs on my bike that might be relevant:

Shifters
Shimano Revo Twist Type

Cog Set
Sunrace CSM66, 11-32, 8-speed

Chain
KMC 8-speed

Front Derailleur
Shimano M191

Rear Derailleur
Shimano Alivio
Sounds like you are getting slack during the ride.
This will happen with new cables but if you get it every ride something then else is going on.   

My thought is that your barrel adjuster is not holding.
Does it look like this?  

  

The barrel adjuster should have a lock nut or something to hold it in place.
Mark the position with tape or a marker and see if it moves during the ride.  

The M 191 is a lower end Shimano but you should certainly expect much better performance than this.

And congratulations with getting up to 20-25 miles a day.