To achieve higher magnification you can use extension tubes, reversing rings and lens couplers.
Extension tubes are the easiest to work with because they have electrical contacts, which connect your lens to your camera. This way you can still retain connectivity and functionality like aperture control. You can have a look at this question: How can I calculate what the effect of an extension tube will be? which explains how to calculate the magnification when using extension tubes. You can also you Macro Bellows which are favoured for there increased precision.
Reversing Rings can also be a very good and cheap alternative. However because they are effectively a 'dumb' adapter you won't be able to use aperture control on your modern lenses. Typically people tend to use old manual focus primes for this as they have a built in aperture ring. The laws of focal length are also inversed when you use this technique e.g. the wider the lens focal length (18mm) the greater the possible magnification.
Lens Coupling rings aren't used as frequently but they can still achieve some fantastic results. Ideally you will want a telephoto and wide angle lens. The primary lens (attached to the camera) is the telephoto zoom at it's highest focal length and the secondary lens is the wide angle set on its widest focal length.
If your really into your Macro photography it might be worth investing in a specialist lens such as Canons MP-E. Or equivalent lens for your camera manufacturer.
It's personal preference what technique you use. However extension tubes in conjunction with your 105mm Macro lens would make for a very powerful setup!