Uploading after the download has been completed (or in other words "seeding") will NOT increase the speed of your other downloads but uploading at a decent speed while you are downloading does help for THAT PARTICULAR DOWNLOAD.
What happens is that most of the good torrent clients like Azureus, utorrent etc. start choking clients that are just downloading and not uploading too well. If you don't upload while you are downloading then you will be choked by other peers. If the number of peers for the torrent is limited then slowly but surely you will be choked by almost all of them hence reducing your own download speeds. Then the only way to get unchoked is to upload well
Seeding or the upload that you do after you have completed your downloads will not help your other downloads but it is the ethical thing to do.