How can I improve my hot water heater's performance?
I had this problem once.  Is the water flow in your shower controlled by a single knob?  If so, then there is an adjustment thingy inside the faucet knob that controls the ratio of hot to cold water.  Take the knob off by prying lose the center plastic cap and removing the screw that holds it on.  There will be a plastic piece underneath that you can pull out, rotate a little bit toward the hot side, and then push back in.  You may have to experiment a little bit to get it where you want it.  

Now, it's been a little while since I did this, and I'm pretty sure I didn't have to turn off the water to tear down the faucet to that point, but I just can't remember for sure, so I guess you should turn off the hot and cold water supplying the shower before you take it apart.  You can probably find an exploded diagram of your particular faucet online if you can still read the manufacturer's name on it.  

This may not fix your problem, but it's a good first step to check before you get too involved with water heaters and pipes just on the principle that it's a 5 minute job and it's free.