Say there's a particular company for which one would really like to work.
How often is it acceptable to apply again for a job, provided you are increasing experience in skills between applications?
As often as a new position opens up or a new round starts for an old position, unless they tell you that it's not necessary for you to apply again for X amount of time (typically 90-180 days).
Just because they hire someone else or didn't hire you last time, doesn't mean they won't hire you this time.
I personally recommend asking the company that you'd like to work with and see how they respond. This should communicate to them that you're not just looking for a job but that you also want to specifically work for them. That means a lot, especially considering that in today's economy the job applicant pool is quite flooded and that many workers are looking for work where ever they can find it. The fact that you want to work for them has value and this makes you more desirable as an applicant.
It's been a long time since I've been on the job market but asking this question has also led to future leads. Internal recruiters and HR reps have always responded by telling me that I could apply as often as I wanted and that this behavior wasn't looked down upon. Additionally, some have informed me of new openings that were going to be available within weeks or months which they felt that I might have been more suited to fulfill. If they are recommending jobs for you to apply for, that's a really good thing!
This type of conversation can some times facility a more informal tone between you and the hiring parties because you are no longer being evaluated as an applicant. Instead, you are simply having a discussion about the company and job opportunities. When the guards of formality are dropped, a rapport is developed between you and the hiring parties. This could mean that when you re-apply in the future, they will remember you, your friendliness and your desire to be with the company. Again, this has worked to my benefit.