In the past two elections I have voted tactically, basically due to the fact that I tend to believe that voting at that level with the current system changes little really, most politicians prevaricate and obfuscate when they arent telling outright lies and the two "main" parties have so little to tell them apart that it doesnt matter who gets in.

So essentially if the current gov has really messed up more than usual or has caused me personal misery directly or indirectly then I vote for the other party most likely to unseat them and if they have done a mediocre job then I will usually vote for the person who is most likely to lose their deposit (you need to encourage the little guy).