Getting the best sound out of your bookshelf speakers is, in part, about getting the right signal to them, but it's also about where you sit in relation to the speakers and how far the speakers are away from reflective surfaces. You can get more volume out of your bookshelf speakers, but it's often at the sacrifice of sound quality.
1Height and Distance: It's important to make the most out of the sound your speakers are producing before amplifying that sound. The first and best thing you can do is to get the speakers at the right height and distance. They should be at the same height as your ears when you are listening to the speakers. Since the higher frequency sounds are much more directional than lower frequency sounds, you will hear more of the high-end stuff if the speakers are pointing at your ears. You should also have the speakers the same distance apart as they are from your chair, so that the speakers and your chair form a triangle with equal sides. Proper placement won't make the speakers actually put out more sound, but they will sound louder to you.
2Reflection: The distance from your speakers to other reflecting surfaces will affect the amount of sound that gets to your ears. But since the high frequencies are going in one direction, most of the sound boost goes to the bass notes. If you place your speaker so it is equally distant from all of the reflecting surfaces around it -- like the shelf on a bookcase or the floor and walls -- then you will get the most boost to the bass, but also the most loss of mid-range sound due to muddiness.
3Obstruction: In the same way that reflection amplifies the sound, obstructions decrease the sound by absorbing the vibration of the sound waves. If you're using a bookshelf for your speakers, make sure there aren't books around the speaker, but especially avoid soft objects like blankets or pillows that will muffle and absorb the sound. If you have your speakers on stands, be sure to use the rubber feet on the stand to reduce vibration and keep all of the energy focused on producing sound.
4More Power: If all else fails you can always add more power to the mix. A bookshelf speaker amplifier is designed to beef-up the audio signal coming in and allows for greater volume at the speaker level. Be careful that you don't over work your speakers or you might damage them. Look at the number of watts per channel that your amp puts out and match that to the wattage ratings for your speakers. It doesn't matter if the amp is slightly more powerful than the speakers, just don't turn the volume up to maximum and things will be fine.