Raising the rear on a model is done similarly to a 1:1 car. As mentioned above, extended spring shackles were very common and pretty easy to create, and as chucky said, air shocks to accommodate the raised height are easy to make as well.
Basically, you need to study the rear suspension assembly of your kit and work out what needs to be done to increase the distance between the axel and the frame. How do the 1:1 guys do it? Extended shackles, re-arced springs, or in extreme cases (mostly trucks) putting the axel below the spring, or on coil spring cars; longer springs or altered spring mounting.
However, on a 1:1 air shocks are used to raise the suspension by forcing the existing springs into an extended condition, but it won't work this way on a model. Air shocks are dynamic but a model is static. On a model the spring leaves won't slide against each other, the shackles won't articulate, and the shocks do not have a compressed air supply to extend them. On a model you will need to modify the suspension to a raised condition (arcing the springs and/or extended shackles) then making the shocks to fit.
A lot of it also has to do with how accurate you wish to be. Keep in mind that the 'cleanest' and teh 'simplest' methods are at opposite ends of the spectrum. If chassis detail is not important, raising is as simple as adding scrap plastic between the spring (either coil or leaf spring) and the frame. If you want precise detail, researching methods for your specific application and developing some scratchbuilding skills may be needed.
Once you get a model on the bench and parts in hand, the problems for your specific application will be come apparent. Then post some photos and specific questions, and solutions to your particular problem can be provided.