Depending on what tools you have access too, how much the dent annoys you and how keen you are.
If it has no "creases" then it MAY pop out with a bit of air pressure, beware popping out "dents" you dont want too. Epxerience is a wonderful thing. You may go as far as taking out the "dents" for your legs or spreading the tank so you need a compressor that will allow different pressures (is it called a regulator?) Just stuffing rags down the fuel filler HAS worked for me before but it helps A LOT is you have a friend (or panel beater) that has done it before.
Once you start cutting holes in the bottom to push the dent out (from underneath) you are looking at least some panel beating and bog unless you are REAL lucky (but then you wouldnt have the dent eh?

)
If you ARE lucky maybe you can get at it through the filler but the chances are slim.
Dont be tempted to try a long screwdriver up the petrol tap hole, you WILL leave a worse mark.