Often Youtube will tack on information to the link, information you really don't need and that makes the link unnecessarily long. Let's look at two examples:
Basically, all you need is the link up to the first & mark, just strip the rest off, as such:
There, doesn't that look much nicer?
Selecting the format:
Some videos will be available in a higher quality/higher resolution. You can tell from the lower right corner of a video having a small button reading either HQ or HD:
Now, all you need to watch the HQ or HD version (High Quality has a higher bitrate i.e. looks better; and HD also has a higher resolution (larger picture)), is to click that icon, but what if you want to let the recipient see that version automatically? Just tack on '&fmt=18' at the end of the link for the high quality version, or '&fmt=22' for the HD version, like this:
and they end up on the better version available automatically.
Linking into a specific moment in a video:
Sometimes you want to show off something that is in the middle of a video. No problem, you can do that too! Just tack on #t=XXmYYs to the end of the video. Where XX is the number of minutes into the video, and YY is the number of seconds. Here's an example:
Note that you can combine these tricks, but the time have to be at the end like this, where we both specify that we want to watch the HD version, and jump into the a specific moment of the clip:
So, there you have it!