One thing I would say about DEET is that it is seriously toxic stuff to put on your skin. It disintegrates clothing if you spray it on it. Of course it's good protection but if you're going for 4 months you don't want to be spraying DEET on your skin everyday.
I used this stuff and found it very effective
http://www.lifesystems.co.uk/psec/insec ... ellent.htmIt has only natural ingredients and they say it's as effective as the 40 strength DEET. In my case I rarely got bitten whenever I used this stuff. They do say however that it's for use in malaria-free zones but in my case I found it effective. Covering up really is key and having a good net (and making sure there a no mosquitos in the net).
One other consideration is blood type. I went in a group of 7 or 8 people and me and one other guy almost never got bitten whereas some people got eaten alive. My girlfriend got bitten all the time and we were in the same mosquito net using the same repellent. It has been suggested that the mozzies like some blood types more than others but I don't know the specifics.
I guess some people are tastier than others.