you do stages 1 and 2, then log out. I did find that doing stage 3 also before logging out isn't that bad. there seems to be enough mobs spawning in the gathering area.. and usually only 5-10 lowbies sticking around.. so at 37 I was still getting like 800-2000 xp per mob.
when you log out (at say stage 3) and log back in, the invasion is reset. So you do all of the stages over again. You don't get the stage clear bonus or the loot boxes, but you still get the decent XP for the first wave, the hefty XP for the second wave (at 38 about 1/2-2/3 level just for that wave alone) and then the decent XP for the third wave. I am usually getting about a level per login doing it this way.
then the two things to remember.. complete stage 4 before you finish or before the invasion goes away (every two hours). Don't want to miss out on that XP. Also, you only have to do this until level 60. Then you get flight (you need both flying and old world flight master) and can do invasions like normal.
Hmm...the relogging thing borders on exploit, no? Has Blizzard commented on this?