Having just had a good interview, I know the excitement and confidence it can create and then the fall which can come afterwards.
The best antidote is also the best practice anyway: don't rest on your laurels.
You can never be certain you will get an offer. It is not just other external candidates, but internal shifts and budgeting which can fill the position or actually reduce it from the head count.
One thing that is certain, with two offers you have an even stronger position to bargain your "end game".
So the steps to take are: get feed back on how it went : asking general open questions before getting to the crux of the matter: how do I stand against other candidates? A knowledge of the time span they are doing first or second interview-round you are in is vital: it is best to not let them off the hook with "we have many candidates yet to see". An initial call to the personnel person present, or the recruiting consulant is worth while because they will see you are keen. Also if you are long off the mark they may well tell you then and there.So then you can get on with job hunting with a little bit of humility if you were trying to stretch yourself, or if you were over competant then you can adjust your sights upwards or undersell yourself a little next time.
When it comes to end phase in interviews, then is the time to ring your prospective manager back. Then you have them in a corner: they have seen everyone, they have come out with an impression on both paper of competance and in person of sociability. Now you have the chance to influence the decision making process.
You should be able to find out your short comings in an objective way. what challenges do you think I would have in the position, or any actual difficulties? DO you think I would fit in socially ? How much better qualified and experienced are other candidates. Be prepared to intersperse these questions with general ones, like how the interview with you had gone, what information may they like to support this , is the start date and so on.
At this stage then you may get a little "puff of smoke" that shows where any shortcomings or mismatches come from. Think quickly: pause and then ask a qualifying, open question followed then by one short closed question. Breathe, then offer a counter point and get agreement to send evidence to back this up.
From this conversation you will get useful feedback from many managers and a feel for your standing at least with managers who go into major avoidance: if they do this then they are covering up for choosing the girl with the biggest bazoongas ! They may just be indecisive or like this situation badly: believe me though, that type of manager is in the minority as in today's flat structured management trees, they didn't get there without being objective and discussing personnel related issues with people.
Move on: do any follow up, and then adjust your CV if you see the need. Be prepared to adjust your whole approach: be more up beat and up sell yourself, or the converse (as here) be coy and undersell yourself. Also if they really doubt you can do the job, then you should talk around with other people to see if this is just one opinion or in fact, you need to aim slightly lower on an experience making job.
Finally you want them to agree to giving you brief feed back in the telephone if you don't get the job. For some managers this in fact can be enough motivation to give you the job when they cannot otherwise see a clear winner.