Christobal made very good suggestions regarding the payment and also the permission regarding your files.
Split your project into milestones and allow a certain number of revisions. The extra revisions should be charged as extra. With this, you'll already avoid the 10-20 round of proofs that kind of client might end up forcing you to do. It's very important if you work for a flat fee.
Sometimes clients who don't give feedback or directions do it because 1) they don't care (or are very busy), 2) sometimes they simply don't know what directive to give you (inexperience) or 3) they think they can do as many revisions as they want. It's usually the case when working with a group of people. If it's the situation you're in, you'll need to make sure the client (e.g. the "leader" of the group) sorts the feedback of his team for you. Otherwise you will end up getting 20 different feedback that go in 100 directions and every single member of the group will want to add their own comment. It's alright if your client has the budget for it but if not, this will need to be limited.
Ideally, it's better to agree on this at the beginning of the project, and one way to make your client agree to this is simply by charging more when you need to work with a team. The reason is that it requires you to manage more people and communications as well than when dealing with one person. And usually, it requires also more rounds of revisions.
For the feedback, you can think of that client as a spoiled kid who wants everything in the candy store but can only choose one thing: Present them a few options without putting too much work on it and go by elimination each time. The goal is to make them say things like "I like X in the proof 01A, and Y in the proof 01B and keep that color from 01C". If you finalize your brochures and show a final result, you might end up having to go through that elimination process anyway but will have spent more time on the first round.
For example, you can start by showing them some drafts of the cover of the brochure only, and use different colors/styles to make them pick one. Then you use the feedback you got for the next round of revisions and make the layout progress a bit more by showing one or a few examples of 1-2 panels. And you do this until you can complete the whole brochure entirely. In other words, you spoon feed them in order to force them to give you some "yes/no" feedback and not waste your time.
Don't be too worried about not getting feedback or directions, sometimes these clients are awesome to work with and very efficient too! Sometimes things get done very quickly actually and it's even better than having clients who micromanage you.
If the client is slow to answer: this happens rarely when you already ask for a down payment and also get paid at every milestone. In fact, you can get paid before every milestone and this will make the client move a bit faster as he will want to get some results for the money he invested. If you only get paid fully at the end of the project, he can drag this for 6-days or 6-months; it won't make a difference to him! The way you can proceed is to send an invoice every week or every 2 weeks no matter if the work has been done or not. That's also a way to accelerate the feedback process ;)
Don't push too much on your client; waiting 2-days is not always enough to get a meeting with their team and get back to you with some feedback. I suggest you let them at least a week or two, and then simply send a friendly email to ask for updates after that.