There are a few methods you can try:
1) Teleconverters. These give you an increase in magnification (equal to the power of the teleconverter) whilst retaining all other lens properties as normal. You keep your min focusing distance and you keep your infinity focusing. Personally I often use a 1.4TC on my macro lenses, 1.4:1 makes a difference in macro and gets you a bit closer, whilst having very little effect on image quality (esp if you're shooting your shots closed down at f5.6 or smaller apertures).
A Nikon 1.7 would also be a good consideration (if it will fit your lens). A 2*TC is harder to work with though - the image quality drop is certainly more noticeable, however 2:1 is also a tricky magnification to work with if you are shooting handheld and I often find shorter working distances actually help as the magnification goes up since you can more easily rest upon the same surface as the subject to shoot.
Note that you can also use teleconverters to retain the same magnifications (just focus less closely) but to also increase the focal length and thus the background blurring and subject separation. I know several photographers who will use 2*TCs on macro lenses for this effect.
2) Extension tubes - these lose you infinity focusing and gain you a reduced focusing distance. No optics inside, but they do move the lens from its optimum positioning so there is a tiny bit of overall loss (generally speaking its negligible - esp after editing). The rough maths here is:
(Length of the extension tubes in mm - divided by - focal length of the lens)+native magnification of the lens = power :1
Eg a 90mm lens with 63mm (a full set of kenko extension tubes) of extension tubes set to the closest possible focusing point (ie lenses greatest magnification) would give you around:
(63/90)+1 = 1.7:1 magnification
Note that the actual magnification might be different because most modern macro lenses reduce their effective focal length as they approach 1:1 magnification on their focusing scale. So in practice I would expect a little more magnification from this setup.
3) Close up lens attachments. These come in two flavours - good ones and terrible ones. Terrible ones are cheaply made and cheaply bought (normally) and are single element affairs. They will work, but will do so poorly. Good examples (such as the Canon 250D and 500D and the Raynox series such as the DCR 250 and DCR150) will give you a much better rendition and, whilst using optics, are very high quality multi element affairs.
These work like extension tubes, but this time the gained magnification increases as the lenses focal length increases - so the opposite of extension tubes. I generally count around 100mm as the middle point between the two - however close up lens attachments come in varying powers (as denoted by their diopter number). The higher the power the greater their diopter number and thus the greater the amount of magnification gain you will get.
Sadly I don't have a quick math method for calculating magnification gain with these.
Personally my tried and used methods are to use a 1.4TC and then if I want more I reach for a Raynox close up lens attachment to add to the combo.
A note on diffraction - most of the above methods will result in effective aperture increase of the lens setup itself. As a result of this diffraction softening will appear to occur sooner in the lenses aperture range. So whilst bare the lens might well shoot up to f18 or further without significant loss of softness due to diffraction; the modified version might well show increased softness much earlier in the reported aperture range. A few tests can easily show you this property and don't feel bad if your selecting wider and wider apertures as you increase magnification - I've often used f5.6 or 6.3 for 5:1 macro to retain sharpness in shots (remembering that the effective aperture of the setup to get there is often well into much higher f numbers - ergo smaller apertures).
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