I'm still working on my first 50 pages plus synopsis. The way I tackle it is to do a run-through of the first 50 pages, then the synopsis, then go back to the first 50 pages. I find this gives me a 'mini-break' and I spot new things to fix each time.
I've also given the first 50 pages to a writer friend (in exchange for 50 pages of his novel) and we swapped critiques this week. It's really interesting seeing how other people interpret your work. Something that you intended a certain way can be read in an altogether different context. Critiquing other people's work is good also because through it you learn to read your own work in a more detached way. (That's the theory, anyway).
Caught up with another writing colleague this week and we swapped notes. It's good having that bit of extra time in the week to do things like this. There's this idea that writers are lonely beings who do it all by themselves. The reality is that, while no-one holds your hand while you write, it's important to have a network of other writers to talk to.