Video is already going to be compressed so compressing it again is not going to do anything.
Trimming the video is a good way though, before conversion though (something I suggest as a last resort) you have several options (someone mentioned muxing to MKV- 50/50 if it does anything in my experience.
1) Losing anything you do not want (extra audio is a good one)
2) Trimming credits or intro sequence- good for many shows as credits and intro are quick long, note though that it might not be a big a gain as you could expect as most encoding tools feature options to drop into and credit quality. On DVDs and newer formats there are usually several things you can happily lose (I find it endlessly amusing seeing people brag about how many lossless formats a given disc has even if I accept lossless vs high bitrate modern codecs/standards has merit).
MKV files can be tricky to work with but once you know how they are by far and away the easiest to play with. A note on file sizes for the respective containers though- it seems to be 50/50 for me if any gains will come and even then it will only be over a whole series or two that any real gains will start to appear.
MKVtoolnix is the toolkit just about everyone uses for this sort of thing:
http://www.bunkus.org/videotools/mkvtoolnix/
MKVextract will errr extract streams from a MKV file. GUI for it:
http://www.videohelp.com/tools/MKVExtractGUI
mkvmerge (mmg.exe is the GUI version) can put it all back together and split files into several if needs be (poor mans trimming really but effective enough that I use it all the time), can also delay audio, subs, set aspect ratio, chapters and beyond).