One tip up front. Do not try to think about two stroke improvement items or tips at one time. Pick one to work on, check it as you swim for a short while, then move on to something else, then come back to that first item and check it again. Trying to think about two things at once usually results in neither one being improved, and thinking about one thing too long can lead to over-analysis and boredom!
This is not really a drill, it is a focal point, a thing to check while you swim. During freestyle, you should aim for the best possible body alignment from the top of your head to your toes. Think a long, straight line, or think "swim tall" with good posture. You will be more efficient if you can swim in a straight line, with your head leading that line. Keep your body inside an imaginary cylinder a little wider than your shoulders. Imagine a tube a little larger or wider than your shoulders. A tube you could slide through head first and have some extra room all around you. Your body should swim inside that cylinder in a straight line, not a curved one.
If you keep your eyes looking at the bottom of the pool, except when you breath, you will be in a neutral posture, and that will help you stay straight. A high head often results in low legs. Avoid that by keeping your head in a neutral position. A good drill for this is head-point drill.
Kick Just Right
Another item to check while swimming freestyle is your kick. Too small and it does not do much for you; it might aid in body balance or in helping initiate shoulder rill, but a very, very small kick will not do much else. That is OK in some cases, and it may be in yours, but do not be afraid to test other kick sizes. The other side of the equation is kick rhythm - 2, 4, 6 beat, etc. What kick rhythm works best for you? Figure that out with a kick test.
Kicking too big and your feet move out too wide, causing excessive drag. The kick may be powerful, but if it is too big, if your legs move too far up/down/right/left, then they are dragging you down.
Kicking just right is someplace in the middle. Your legs will generally stay within a space a bit larger than your shoulder circumference. If your kick gets too big, it hits the sides of that tube. A good kick is big enough to almost hit the sides, but not so big it would crash against the sides of that tube. Head point drill can help you figure out kick size; as you do the drill, try it with different kicks and see what happens!
Roll Your Shoulders
To swim good freestyle you have to engage your pectoralis and latisimus muscles for the pull, and you have to recover your arm above the water. If you lay flat in the water, both shoulders staying at water level, it is very hard to properly do either one, and it is hard to turn your head far enough to get a breath. By rotating or rolling your shoulders (and chest/trunk/waist) you can get your arms in the right position to engage those big muscles, and you can get your arm out of the water for the recovery without getting a bad case of swimmers shoulder. Head point drill is also useful for working on body rotation or body roll.
Try thinking about these - or other stroke improvement tips - during parts of your next workout.