Assuming you're using iron sights, have a mechanically-sound firearm, and are getting too-large groups from standing, sitting, or prone (as opposed to off-the-bench), here's some suggestions:
1. Concentrate on sight alignment (keep the front and rear sights correctly lined up);
2. Focus your eye on the front sight, so as to keep the front sight sharp and clear, while letting the target and rear sight go fuzzy;
3. Work on trigger control :
a. squeeze the trigger at least two seconds (do not snatch or jerk the trigger)
b. hold the trigger back when the rifle goes off
c. keep your eye open and take a quick mental snap-shot of the sights/target when the rifle goes off, so you can call the shot.
These would be the first things to work on when your group is OK, except it is simply "too large" - which is anything over 4 MOA, right?
If you have trouble in prone with the sights wobbling about and not staying on target, pull up your trigger knee as tight as you can behind your trigger elbow - you should see an instant 'tightening-up' of your position, and the front sight steadying down on the target.
Not least, 'do your homework' - get into prone three times a week and dry-fire (by the numbers, of course) ten shots. This will help speed your body getting comfortable in prone, and will make a real impact on your shooting performance when you get to the range.
A proper trigger squeeze should be like breaking a tiny glass rod.
Just gently squeeeeeeeeeeeeze it back until the rod breaks. You should not be able to predict when it will break.
The initial post assumes your groups are BIG - like 12 - 20 MOA or bigger.
If your groups are smaller, it's much simpler, in a way. Simply focus on 'polishing' all the basics.
I've seen a number of times Appleseeders who can't get it together - who start out with 12" groups at 25m (!) and stay there all day Saturday.
But they stick with it and, sometimes late Saturday, but often on Sunday, it finally 'clicks' and their groups decline dramatically - to 5-6 MOA, sometimes.
In other words, they're down to the 'polishing' stage.
Once again, you can't work on your one inch squares if your shooting in a 12 inch box. Work on this before you get to where you are going, or before the convoys arrive anyway. ;D