You can do quite a bit. Although, there gets to be a point where you might be better off just buying a new bag and being done with it.
For starters, make sure youre well-dressed, head to toe. Midweight long underwear (REI Midweight MTS top, $35, and bottoms, $30; rei.com), plus gloves, hat, and warm socks (Smartwool Light Hikers, $15; smartwool.com). Theres nothing at all wrong with adding to that a fleece jacket or anything else. Insulation is insulation. The only caveat is not to over-stuff your sleeping bag so much, with you and all your clothing, that you press against the inner shell and compress the insulation.
Second, make sure youre well-fed. Eating a candy bar or something similar just before bedtime (nothing with caffeine, obviously) will give you a few extra calories to keep warm with during the night.
The coldest spot is always the ground, so make sure youre covered there. Double up on your pads. If you have a good inflatable pad such as a Therm-a-Rest Prolite 4 ($85; thermarest.com), supplement it with an inexpensive Ensolite (closed-cell foam) pad for $15 or $20.
Finally, you can add a liner to your bag. Keltys Fleece Sleeping Bag Liner ($45; kelty.com) will add about ten degrees to the low end of the bags comfort end.
Or, you could just go out and buy a North Face Cats Meow with Polarguard insulation and a 20-degree temperature rating for only $159 (thenorthface.com).