In the past I have used a travel agent, but not anymore as the one i used retired and now a days it doesnt save any money. The travel agent i had was a family friend who when they retired passed along the tricks of the trade and how/where to look on IATA and others to find the lowest prices on air/train tickets, rental cars, hotels, etc. I book and manage my own travel but I also do some planning and arrangements for some friends and family but that is it as I have to many other things going on.
Considering between work and personal trips I do a fair amount of traveling (50K-150K air, 25-75K land, and water (variable distances)), and have for the better part of 15+ years (lots of Butt in Seat time/miles, and tons of stamps in the passport).
So over time i have learned alot of the ins and outs of the different loyalty programs that i have status in (which really helps with things go wrong) as well as keep a eye on other programs to see if they would meet my needs. Alot of people dont think about the points and miles side of things but over the years that has been a nice perk along with the program elite status as i have used those points and miles for tickets/rental cars/hotel rooms that would have cost tens of thousands of dollars if it was out of pocket. For example NYC - Sydney in first class with the routing of JFK-LAX on American LAX-SYD on Qantas, then SYD-HKG-JFK on cathay pacific, retail on that ticket is almost $35K but was only 145K miles. Then on the hotel side of things i have used points at hotels in europe and far east that cheapest room is $750+ a night but were only 25K points a night. Theres alot more to it but thats a subject for a different thread.
as for planning alot of the time if im going somewhere just to photograph or adding on days to shoot i will contact people in the area i know as well as look online for items of interest or vantage points. I also use google earth and a few gps programs to get fixes on locations. I also keep extensive notes on places i have visited in the past as well as information i have heard form others i have worked with or know. Even with the trips that are planned extensively I always have a camera on me just incase i see something that grabs my attention as I have had a couple of portfolio images come from having a camera on me and being at the right place and time.
as for traveling and photography if your looking for travel tips i would suggest Steven Frischling and his blog Flying with Fish http://boardingarea.com/blogs/flyingwithfish/
This blog is a good resource for packing as well as traveling tips with camera gear. I travel alot but steve has me beaten hands down as I used to do assignments like that but not anymore. I picked up alot of tips from steve, but i have also adapted a system where between a std 22" rollaboard and a backpack I can go for 2 weeks and shoot anywhere (outdoors in natural light / long exposure to studio conditions - including, tripod, lightstands, speedlites, pocketwizards, modifers, etc) as well as clothes and still have room to bring home a few things. The set up isnt light but I know how to make it look like it weighs nothing.
YMMV and IMHO