sta travel are pretty good as all their sales-people are all/ have been packpackers themselves and are fairly knowledgeable about pretty much everywhere on the standard backpackers route.
we went in with a list of countries we wanted to get to and they worked out the best approach as to where to start, which direction to go etc.
Trailfinders are also handy, but we found that they were slightly less personable people and that they were slightly more expensive to.
Also remember to make an appointment with your practice nurse re vaccinations etc. required, and to check visa requirements for each country you are going to. If you have a British passport then its fairly straightforward for most countries, and you can get them on arrival but there are a few anomalies.
One last thing to bear in mind is travel insurance
best of luck with it, you'll have a top time. Wish I could do it all again.