We ended up tweaking most of our combat systems whereby we have an initial initiative role at the beginning of combat and then if your character performs a successful action (i.e. successfully hitting an opponent or casting a spell), without being hit for that turn of combat, you get to roll a D10 and add it to your initiative.
It kind of mixes things up a bit and encourages people to be more assertive in order to up their initiative for the next turn. We also added a rule that lets you decide to forfeit you extra initiative for a turn in order to lower the initiative of an opponent, which also works pretty well.
I think we started using the system with L5R initially, but since then we have used this initiative system in all the games we play. Give it a try, it works pretty well and makes things a lot more interesting
