This explains trimming, a little. Basically it is clicking the levers in lightly so as not to move them the entire distance of a full shift. It only moves the derailleur a shorter distance than the full shift. It can be used to help avoid chain rub and it makes the next click of the shifter engage the shift easier and quicker, racers and other high performance riders use it mainly I think (as well as cross chaining clowns like myself..I am breaking that bad habit slowly but surely
). A lot of newer riders are not really that aware of it I think.
I had to go out for a while but I was thinking that maybe your cable might just be stretched a little if it has never been adjusted since you got your bike. First thing I would do is see if you have an inline barrel adjuster for the front derailleur and try adjusting that IN SMALL INCREMENTS (1/4 turns) and see if it takes enough slack out of your cable to help. Don't get carried away with the turns.
If your bike was new when you got it and has never been adjusted since, there is a good chance I think that could be the problem. The above video is a good one and I would even suggest looking at others on youtube as well. If I didn't feel comfortable enough trying to adjust it myself I would take it to a LBS and let them check it out. I would try to watch them if possible.