I've had quite a few people now contact me all with a a similar problem. They get a lot of people excited about the idea of a group trip away together but when it comes time to commit they have problems getting those same people to front up with the cash.
Essentially this is the biggest problem you can face. Nobody has had a problem coming up with a trip that excites people or finding a travel company to deal on that itinerary. The blocker is just getting friends and acquaintances to actually fork over the cash when the time comes. Most frustratingly is that it seems many people make the right noises right up to the due date and back out right when you need to get the cash across to the travel company to make it all happen. At this point it is too late to find replacements and the whole thing goes bust. This is not only disappointing for you but probably even more so for those who like you were fully committed and totally excited. Will they trust you again to organize something?
I have too different strategies to help you alleviate these problems. They are both about creating transparency in the process.
The first is probably pretty obvious but you need to create an online place for the group to communicate so that at worst, committed travelers can see exactly the level of commitment from others in the group, as opposed to emailing, phoning and texting around to everyone individually. Contacting people individually as the only means of communication puts way too much pressure on you as the organizer to convey everyone's level of commitment to the rest of the group. The simplest and most effective method is to create a Facebook page for the trip and invite people from there. Once people "like" the trip they will get updates from you when you post info into the page status update right into their regular news feed. This allows you to continually touch the group and keep the idea fresh. Encourage others to post their thoughts so there is a public record of enthusiasm or otherwise. The page also creates scarcity. If there are 10 spots and 100 people 'like' the page then in theory 90 are going to miss out - which can induce a quicker commitment to the available places.
The second tip is to put the trip up on OBaz . OBaz is an online resource that allows you to put your trip online and receive payment - including credit card payment - for the trip funds. There is only one type of commitment that means anything in the end and that is cash in the bank. Travelers pay their funds and secure their place. Make it clear that you can't hold places for anyone. First 8, 9 or 10 with cash deposited are in. Everyone can go to the private group record to see who has paid and how many spots are left. You can also post this up into the above Facebook page, which saves you from the endless and inevitable questions of whether the trip will run or not. The facts are all there for people to see, online and delivered to a place they visit regularly anywhere - their Facebook page.
Hope this helps get you out on the road for Free!