I LOVE JAMAICA! Which part are you going to?
I'm getting cornrows again tomorrow, this is only my second time having them with extensions, but I will tell you what worked for me last time.
I washed my hair/did a light protein treatment/deep conditioned the day before I got the braids. I also applied a moisturizer to my ends the night before. If your ends need to be trimmed it's best to do that before you get the braids.
Thats all I really do, the girl who braids my hair told me you're not supposed to grease your scalp before you get braids because it will be harder to grip the hair...
[editing for during] I think it's very important to keep your hair moisturized while in braids. It's easier if you use a braid spray for this. I use Sta Sof Fro, but my sister uses Cornrow Magic and likes that. Some of the ladies on here rave about Better Braids, Sulfur 8, or African Royale braid spray.
You can clean your scalp with SeaBreeze, or just rinse your braids with water every few days. I do either one, I don't use shampoo or anything to wash my braids. When I take my braids out I don't have any buildup. HTH