Well, the best way is to limit torque on low revs. 300bhp does not kill your gearbox, but 500nm torque with 2000rpm does. You may have to use bigger turbo to achieve this goal, but something can be done with appropriate trionic software.
I know a lot of guys who can destroy anything ( no, i'm not one of them ), so try to drive 'wisely'. One should be aware, that saabs ( orig. gearboxes ) with massive power levels ( > 300bhp ) will have gearbox problems sooner or later. This is a fact.
Limited slipp differentials can help to protect your gearbox ( by restricting excessive wheelspin ) as well. ( See for example
http://www.speedparts.se/prod/sp_vxl_diff.shtml in sweden or
http://www.quaife.co.uk). These units are quite expensive, but they will make your saab a lot faster.
Note also, that stronger clucthes can manage high power levels, but these will also put some extra stress on your gearbox. I think that better ( wider ) tires have this same effect as well. So, by using 'normal' size tires you will have some extra wheelspin, but this also protects your transmission.
Then, if you are a rich man, you can buy better parts for your transmission. See for example
http://www.quaife.co.uk/ . I think this is the only way to go if you really want to avoid gearbox rebuilding at least once a year ( with 300bhp++ powerlevels ). These guys can help you, but it will cost a lot.