One thing that every brewer should have just waiting for action is a bit of dry malt extract.  Not every brew session hits your specific gravity quite the way you expected and having a little bit of dry malt extract around can help boost up your gravity to where you wanted it to balance out with your hops.  Having your OG:IBU ratio is important and when you come up short on OG, it throws the balance off.

So, keep an extra pound of light dry malt extract around.  When you are coming up to the end of the boil, take a specific gravity measurement with a few minutes left to see how your gravity is coming.  This is really easy if you have a refractometer.  If you are doing a partial-boil extract batch, you need to factor in your diluting water to the measurement.  Final Gravity = End of Boil Measurement * Boil Volume / Final Volume.  If your numbers are low, you can quickly calculate how much DME you need to add in, boil it for a few minutes, and be "extractly" where you want to be.  

A pound of DME will add 44 points per pound so if you mix a pound into 5 gallons, it should add about 9 points.  So, for easy calculations, for each specific gravity point that you are low, add an eighth of a pound.  If you calculated that you were at 1.054 and you wanted to be at 1.059, add in 5/8 of a pound and you should be okay.