this is from the wiki, to the best of my knowledge this is still current info...
In very rare occasions, it's possible to be promoted twice after one Series, due to having a high MMR. For instance, someone in Gold V that duo queues often with his/her friend in Platinum III might have an unusually high MMR due to the players that he/she is matched against. As a result, his/her MMR may be 1 or 2 divisions higher than the average MMR of their current division. After winning their Promotion Series, there is a possibility that he/she may be promoted twice. Dodging queues may also, to a smaller degree, affect the difference between your MMR and LP (-3LP per first dodge of the day, -10 thereafter).
As of Season 4, it is possible, however rare, to skip promotional series. You will need to have an MMR one tier higher than your current placement. For example, if you are Gold V, so long as your MMR is Platinum IV or higher, you will instantly be promoted upon reaching 100LP. It is possible to skip divisions along side skipping promotional series.
http://leagueoflegends.wikia.com/wiki/League_System