It is helpful to have a passport and ticket holder to wear around your neck, as you will have to show both multiple times in the airport.
Hanging toiletry bags are very convenient; one for bath & hygiene items and one to keep all your small loose stuff. There is very limited floor space to be getting in and out of your suitcase, and these can be hung in the girl bedroom or the communal shower.
Noise canceling headphones can help keep you sane on the plane.
Small earplugs may be needed for some of the church services. Yes, you will still be able to hear just fine with them in. trash bag for dirty clothes is a good idea, as they will get pretty moldy in the high humidity and you do not want them touching anything in your suitcase.
Bring extra batteries for your camera and flashlight in case you cannot use your rechargables.
An LED flashlight has much longer battery life. Also a small lantern style flashlight is nice for use in the bathroom.
LL Bean makes a nice travel bath towel that dries very quickly, packs thin in the suitcase, and is antibacterial. Current name is Aquis Adventure Towel.
Drew and I like to plan our outfits for the week and then layer them in the suitcase. Every day we just put on what is on top of the pile without having to dig through the suitcase. You will wear the team T-shirt for the fourth clinic.
Pack your bedding, flashlight, presents and sleepwear on the top of your suitcase items for easy access when you arrive at the house in the dark.
I like to keep my essentials for the daily clinics in a small fanny pack: a couple of wet wipes, toilet wipes, a little toilet paper, chapstick, a pen, a small memo notebook, some gun, and a granola bar. Extra wipes, your Bible, water bottle, and raincoat, sunhat and sunglasses can be kept in your backpack.
Keep an older pair of dirty tennis shoes to wear down. They will get even filthier.
I like to keep a set of older, somewhat ratty clothes to wear on all the trips. Then when I get back I do not have to do a laundry right away to have something to wear.
Remember the new air travel rules on carry on liquids: 3-1-1. You are allowed 3 ounce bottles, in 1 ziplock, of a 1 quart size.
I like to take my raincoat as a carry on because it can be pouring when you arrive in Guatemala. And it can be pretty cool on the drive if it is raining.
Bring a few empty ziplock quart and gallon bags. They will come in handy.
The water is really soft. You may only need shampoo and not conditioner. Ditching the conditioner will get you out of the cold shower faster.