how to use arxiv? This is a soft question. How do people usually use arxiv to put their papers? At which stage does one usually put his/her paper/report there? Someone suggests me to submit a paper while putting it on arxiv. Is that the convention that people follow?
Thank you!
Anand 
 A: What I personally do is as described by Emerton in his post, and so the following is what other people have told me, and not perhaps totally my own thoughts.
I have also heard that people do the following: post to the arxiv after acceptance by the journal but before giving the final proofs.  Advantages:


*

*You can say "Accepted by Journal X" and so you won't "lose" any citations.

*If the referee asked for major revisions etc. you don't have the "embarrassment" of submitting a major update to the arxiv (if you care, then remember that you cannot delete anything from the arxiv, only update, so everyone, if they look hard enough, can see you mistakes, as it were.  I have definitely had people say to me that this puts them off using the arxiv).

*Your work is still made accessible to everyone, and more quickly than in journal (especially if Journal X has a long waiting time).


Disadvantages:


*

*You're not getting your work out in as timely a fashion as possible-- the gap between submitting the work (i.e. when you finished the work) and the referee getting back to you can be ages (in my experience).

*You lose the possibility to get feedback from the community.

*You are perhaps treading a fine line when it comes to "ethics"-- you're not really posting a "preprint" any more, but basically the final version of the paper (sans any minor typos caught in the proof stage).  I can see that publishers might get annoyed by this (but have no evidence that this has ever happened).

*You lose the "public timestamp" feature of the arxiv-- hypothetically, one can imagine getting rejected a couple of times by slow referees, and so the gap between finishing the work and acceptance (and hence posting on arxiv, to make it public) being over a year, and this, again hypothetically, could lead to a priority dispute which would have been avoided if the finished work had been made pubic immediately.

A: My comments above formulated as an answer:
People typically post a preprint on the arxiv at the same time that they post it on their own homepage, with the goal of disseminating their work to their colleagues. (These days, posting on the web is more important than journal publication for sharing your work, and the arxiv is the central repository for math on the web.) This is often at the same time that they submit to a journal, although sometimes they wait for feedback (as Joe Silverman suggests in comments above), and sometimes they spend more time polishing their preprint before submitting it (as Darij Grinberg suggests, again in the comments above).
The conclusion seems to be that it is standard to post on the arxiv as soon as one is ready to share one's work with colleagues, and that this is often at, or close to, the same time that one is ready to submit one's paper. In particular, it is quite common to post on the arxiv at the same time as submitting, or not long prior to submitting.  (But there is nothing wrong with posting on the arxiv and then spending some more time polishing your preprint before submitting it to a journal.)
A: Yet another way in which people could use arXiv is as a repository for material which otherwise cannot find a home in a journal.  Sometimes in the course of working on a project I wind up with some material which did not make it into the published article -- or perhaps some notes which record my growing understanding of articles already published by others -- which look like they could be useful to the community as expository or supplemental information, but which in my opinion are not otherwise significant enough to warrant submitting to a journal.  I have sometimes wondered whether it would be appropriate to post such material on the arXiv.  (I have not done so yet.)  For that matter, I wonder whether others have done this very thing.  
One drawback of this use of the arXiv is that everyone knows that most arXiv articles have not been peer-reviewed at the time of first posting, so they must be taken with a grain of salt. If an article is never published in a journal, you must read the arXiv article with a more critical eye.  But as I said, I do believe that there exist some notes that are perhaps worth sharing but not worth wasting the effort to peer review.
