A true trainer doesn't need to use glitches :p
There is one I heard about where if you move back and forth really quickly on the bike you can sometimes cause the maps to stop loading, it's just black. Now you can go riding into the darkness and go to anywhere like the island where Darkrai is and the field where Shaymin is, without needing the event items.
There's also one I saw the other day. If you give a pokemon with 1HP a Pomeg Berry, it will have -1HP. Then if you get hit, your HP keeps going down and down and when it gets to 0 it resets and goes down again. You can also battle using an egg. Apparently it works on Emerald and all later games. See this video: