Once you've passed the qualifying exams, the next big blocker for most people is the thesis. Don't let ABD be a terminal degree.
Outside the experimental sciences, budget one year for the initial draft of a thesis, no more. This includes all research. If your thesis topic is too big to research and write in a year, cut it down until you can get it done in a year. The goal of the thesis is to tick the checkbox on the list of PhD requirements, not to produce a world changing work of scholarship. You can always keep working on the subject after your PhD and publish a book for that.
Akrasia, procrastination, and writer's block are big problems for some students at this stage who've never had to be this self-directed before. There are plenty of posts here that address those issues. If this is happening to you, read those posts.
I recommend writing from the outside in. That is, start writing a very rough outline almost immediately. Fill it in and flesh it out every day. Write something, and if you aren't sure whether the something is true, write it anyway and put in a TODO comment to find out. Mix your research and writing. That is, do not plan on 6 or 12 or 18 months of research to be followed by actually writing. As you research, your thesis should be filling in and getting more complete day by day. When you're finished researching, you're finished writing.
If you discover that one chapter of your thesis is growing out of bounds, consider whether you can cut everything else and make that one chapter the entire thesis. E.g. if you discover that your thesis on the development of Ottoman Empire bureaucracy from 1718-1914 is bogging down in the Napoleonic era, maybe the thesis should instead be the bureaucracy of the Ottoman Empire in the Napeolonic Era. Or maybe it should be the development of the Ottoman Empire bureaucracy from 1718-1799. Either way, cut your topic down to size at every opportunity.
I have less experience with theses in experimental sciences; but my impression is that in experimental science, the real thesis is a pure-reviewed paper significant enough to be published in one of the major journals of the field. This paper is then expanded with a bunch of supporting details (tables of experimental data, computer code, etc.) that couldn't fit in the journal article. Once the research is done and the paper accepted for publication, producing the dissertation itself should be relatively easy. I don't know how long it should take to do the research. However, I do recommend aiming for the most obvious approach with the least risk of failure. E.g. rather than trying to develop a new experimental technique for measuring lipid profiles that may or may not work, prefer a thesis topic that involves using well-established techniques to measure the lipid profiles of some undersampled population. For instance, repeat an experiment that's already been done, so you know in advance how long it will take and how much it will cost, but this time do it on women instead of men, or senior citizens instead of college students.
The unifying theme here is that you need to limit your investment in the thesis and avoid procrastination. Find the shortest path to completion. Do not make the mistake of trying to fit a topic worthy of an entire career into one dissertation. Plan for a year from topic approval to completion. If that's unreasonable, find a smaller topic until it is reasonable. If your thesis takes longer than planned, that's OK provided you're making regular progress and the end is in sight. An extra year won't kill you; but if the end is more than a year in the future, something has gone wrong. Back up, reevaluate, and cut your topic down until you feel that you are likely to finish within a year.
There was some support for the idea of starting an advice repository for grad students much in the same tradition as the Boring Advice Repository and the Solved Problems Repository started earlier by Qiaochu_Yuan. So here goes.
Please share any advice, boring or otherwise, for succeeding at grad school. I realize that succeeding might mean different things to different people, but I believe most people largely agree with what it means in this context. Feel free to elaborate on what you believe it should mean, if you have views on the subject.
I am a theoretical physics grad student, so I'm personally more interested in advice for mathy disciplines (i.e. physics, math, CS), and I also suspect that there are many grad students from these disciplines on LessWrong; but advice for any discipline is welcome as well.
Advice is welcome from anyone, but please do mention your background for providing the advice so that people can weight the advice accordingly. For example, I would be more be open to listening to advice from someone who has completed a very successful PhD, than from someone who has simply interacted with a lot of grad students but has never been to grad school.
Also, feel free to link to advice from other sources, and maybe quote the most useful parts in what you read. Remember, this is meant to be a repository, so that people can come and find the advice, so don't worry if it seems to be something most people would've already read or known.
Thanks!