I have the same issues. I currently use the Activity Locking Plug-in. I'm using the version posted by Andrew Chow in the discussion: http://moodle.org/mod/forum/discuss.php?d=92731. I did have to do some tweaking for Moodle 1.9.2, but it works well. There is a more recent version (downloadable from that same discussion), but I haven't installed it yet. It may addressed some of the drawbacks I list below.
You basically have three options: a student must view certain resources before accessing assignments (great for instructions!) and/or they must obtain a minimum score on select assignments before accessing locked assignment. (If I don't really require a minimum score, I put '1,' since 'minimum score' on prior assignment and/or accessing a previous resource is the only way to lock it.)
I consider the few drawbacks minor, given what the plug-in does accomplish. It does not work (or work well) with AJAX enabled. Not an issue for us now, but it will be eventually. Also, when a teacher tries to grade or access (not necessarily edit) an assignment, he/she is sometimes subject to the same prerequisites (usually if the prerequisite was a resource). Not a major deal, just a bit of a pain.
I recommend trying the lastest version.