Pull the shifter out and check to see if the plastic bush is still on the end of it. If it's gone replace it.
Also, where the selector cover is, just below where it bolts on the is a 24mm bolt either side. Pull both these out and you will find a couple of springs, chuck an extra stronger spring in there and it will make it firmer. If you check out gktech.com.au, he sells the springs and extra bolts for crazy cheap. If you purchase then, use your standard bolt as the gktech ones are to suit sr boxes and drill out the standard bolt to accomodate the bigger springs.
It works as i've done it to mine for when she's ready to be put in the car. Hope this helps