Here are my thoughts, some of them are similar to some of the other suggestions already given. These are in no particular order though I think 2 is my favorite, but I like 3 a lot as well:
1) More Encounter Powers should be given to martial characters (fighters, knights, rouges, etc.). Instead of dailies give them 2 or more encounter powers to choose from of roughly equivalent value if they were combined into one power. This ensures fighters will have lots of interesting attacks to deal out throughout the course of an encounter.
2) Create a system similar to power strikes or psionic power points where martial characters have a set number of points to spend on actions that can be used after a hit. For example, Essentials martial classes all have the power strike encounter power instead of dailies. I think power strike as-is is actually very dull, but a lot of the options for power strike if you spend feats on them (i.e. a waste in most builds due to them crowding out other more useful feats if taken) you can use your power strike to do all sorts of additional maneuvers and attacks on a hit. For example Heavy Blade Power Strike (FEAT) lets you spend a power strike when you hit an enemy to roll your power strike damage die (1w) against another enemy adjacent (similar to how cleave used to work prior to 4e).
To improve on this have a list of maneuvers that fighters can preform after a successful attack by spending a point. They can use them as much as they like without any limit other than the number of points available each encounter and them needing to successfully land an attack first.
3) Create a group of At-will minor actions for fighters to use as this would give greater utility to the martial classes and help to separate them from the magic classes. In 4e minor action attacks and utility powers exist but mostly as dailies or encounters and even then most characters only have a few throughout their adventuring career. Giving martial powers at-will minor actions helps to mechanically and flavor-wise set them apart from magic classes. Fighters are men of action, looking for openings in their opponents and never letting up, this would help simulate that.
4) Create a combo system fighters and other martial classes are able to chain attacks together much like fighting games on video game consoles. A successful initial hit on an attack allows a followup attack. These follow up attacks in the combo could be totally different attacks and indeed ones only useable after the first combo starter hit.