Hi Fraser
David has given you some good advice above.
I would just add that you need to be quiet and observant. Walk much much slower than you normally would and scan around well in front of you. You can see reptiles any time of the day but I agree with David that early mornings are better....animals will often be lying outside their hides(logs, crevices, tussocks etc) soaking up some sun. Once they warm up they are much more likely to dissappear if they sense you coming, often before you even see them.
I regularly use a pair of binoculars to scan good looking areas in front of me....but it is still very easy to overlook things. I get a great buzz out of looking somewhere and finding things just with the binoculars. If you decide to buy a pair sometime...make sure that you get ones that will focus closely...mine will focus on something only 1.7m away so great for watching things.
Don't go blundering through the thickest bush or longest grass looking for things....anything that's there you will only scare away. Try and follow a track or natural openings.....reptiles will often use these as well as they offer access to the sunlight. Mixed or patchy environments are also very good......where bush meets farmland, where a rock outcrop meets a river etc.
After a while you will get a feel for the "right sorts of places" but then reptiles can and do turn up just about anywhere to surprise you!
Also never go herping without a pressure bandage in your pocket, and if you are looking under cover(rocks, logs etc) always put them back exactly how you found them.
good luck and we expect to see some posts of your trips!
cheers
Greg