Often I've come across rules of thumb about maintaining active publication record. The advice I've most often been given is that one should have at least two papers under review at all times? What's your strategy? I am in the social sciences.
You're simply asking the wrong question. Your goal should be to do the best work you can, not the most — quality, not quantity. One revolutionary paper is worth far more than dozens of incremental papers that nobody reads.
To put it another way, if you find yourself submitting so many papers that you're worried about being overwhelmed, you're doing something wrong. Aim higher. Publish fewer, bigger chunks of work. Spend less time writing and more time thinking and digging. Stop just writing papers and develop a research program. The CV bullets will take care of themselves.
Saying you should always have N papers under review is foolish. Say N = 2. You've just submitted a paper, and then a few days later, the other paper comes back from review with an acceptance notice well ahead of schedule (in two months instead of four). Does that mean you have to submit another one immediately, in order to have two in the pipeline? That doesn't make sense to me, as it makes your work subject to arbitrary deadlines that are orthogonal to producing high-quality publications.
You should keep a general track of how the work in your group (or your own work, if you report to someone else). You should also keep track of the general expectations of someone at your current career stage, and make sure that your publication output is consistent with your peers. (Don't worry about exact agreement: producing 80 or 90% of the total output is not really a big deal; producing 25 to 50% is likely to be an issue.)
Beyond that, however, you should focus on making sure your publications have quality, with a much lesser emphasis on quantity. One Nature or Physical Review Letters (or equivalent journal in your chosen field) article is probably worth a lot more than two or three publications in other journals.
JeffE's answer is excellent, and probably the most "correct". Nevertheless, as food for thought I offer the following anecdote from Bayles and Orland's Art and Fear:
The ceramics teacher announced on opening day that he was dividing the class into two groups. All those on the left side of the studio, he said, would be graded solely on the quantity of work they produced, all those on the right solely on its quality. His procedure was simple: on the final day of class he would bring in his bathroom scales and weigh the work of the "quantity" group: fifty pound of pots rated an "A", forty pounds a "B", and so on. Those being graded on "quality", however, needed to produce only one pot - albeit a perfect one - to get an "A".
Well, came grading time and a curious fact emerged: the works of highest quality were all produced by the group being graded for quantity. It seems that while the "quantity" group was busily churning out piles of work - and learning from their mistakes - the "quality" group had sat theorizing about perfection, and in the end had little more to show for their efforts than grandiose theories and a pile of dead clay.
I somewhat agrees with JeffE, but on the other hand it's not like one can just decide to write a ground breaking paper.
Having a steady stream of visible work is something positive to get a job, and one should neither publish dozen of small papers nor stall on a very important problem he cannot solve.
One way to get a good balance is to aim for a given stream of papers : if you get them easily, then try harder problem, if you don't, then maybe you should try something more manageable. Of course, what you aim at is very dependent of your field (and even sub-field).
I am in fundamental mathematics and personally like to have, at any given time, at least one paper under review, one paper that mostly needs writing, one idea to develop that could make a paper. In that way, I can focus each (non-absorbed-by-other-task-than-research-)day on what I feel like: writing, reading to back up the idea, do the needed computations, etc. Moreover, having a submitted paper makes me feel more comfortable trying things that take time but have a good chance to fail.
The advice I've been given for cognitive neuroscience is to (1) make sure I have at least one publication every year, (2) invest the rest of my time in high quality work.
The reasoning is that a full year's publication gap in your CV looks bad, but on the other hand, what people mostly look at when you apply for jobs is where you published the handful of your best papers. One or two high-impact publications are more likely to impress people than ten low-impact ones, but a publication gap looks bad regardless of where your best work got published. How many papers you should have under review at any given moment to fulfill this goal will depend you your research area and the duration of the review process.