A major cause of problems with Glocks is improper lubrication, primarily over-lubrication. Glocks need very little lube in order to function properly.
A good guide to lubing the Glock can be found at http://glockmeister.com/glubric.shtml
One point that I want to emphasize is the statement that reads:
The most important drop of oil is placed where the trigger bar and connector meet. Failure to properly lubricate the junction of the trigger bar and connector can lead to premature wear and a very heavy trigger pull.
If you never lube any other location on the Glock, be sure to periodially put one drop - and no more than one - at this location.