I second taking a look at Tasker. It does so much that you can't even explain all it can do for you.
Notification specific, take a look at LightFlow. The free edition is usually good enough for most people. It can change sound, light color and vibration on your notifications. It can change if they happen at all, how often and for how long (i.e. every 5 minutes for the next 2 hours). You can even change how often the light pulses. Best of all, the notifications can be for what (missed call, low battery, new MMS) or for who. (Note: the different light colors is hardware dependent. Not sure what your HTC is capable of)