How do you sharpen a pencil without a pencil shapener?

I hardly ever use a sharpener to sharpen pencils, actually, because it's hard to get a good point. Get an Exacto knife (I think that's how you spell it), and scrape off the top of the pencil equally, going all around the pencil. If you've done that, but the tip still isn't sharp enough, just scrape off the lead, not the wood underneath it. Here are some more in depth/other methods:

Sharpening with a knife

A knife is the most versatile method of sharpening a pencil. It can take a little practice but it's a very useful skill to master. Make sure you cut away from yourself and preferably over a bin. With softer leads try cutting smaller bits at a time to prevent breaking.

Using sand paper

This is a neat little trick that I use regularly. Try clipping a little piece of sandpaper to your drawing board and if your pencil starts getting a little blunt just run it over the paper a few times to bring the point back. This is especially useful during quick figure drawings when there is no time to get out your sharpener.

Paper wrapped charcoal

This is a very neat way to sharpen charcoal pencils. Simply pull back the string a little then peel off a roll of paper. The best thing about this method is you are left with the 'bullet point' style of sharpened pencil, very nice!

Scissors

You could also use scissors. Just simply grab any scissors (any kind) then just cut until it's sharp. I have tried this myself and it can get very sharp
You can use a knife but a sharpener is best
You can use a pocket knife (or any sharp carving knife, for that matter) to define the point of a pencil. Although, achieving a good point is difficult, so a sharpener is preferred.
Use a knife. It works.