I'm hardly a professional letterer, but part of my day job is to work with lettering files for comic books.  I picked up a lot of really fun and informative tips and tricks by learning from the pros!

Lettering's important!

When we pick up a book, we want to be able to read them very well.
There shouldn't be anything in the way that should make us go:

"Huh? That's hard to read!"
"I don't know what that says..."
"What should I be reading first?"

It's our job as comic book artists to draw our readers in, and try not to break their concentration as they're enticed in the worlds and characters that we draw!

So here are a few tips:

- Use the same font, same size, for the majority of your dialogue.

Why?  This way, when you need to change sizes, you can show... emphasis.

I mean, it would be really weird, if let's say, I'm writing this tutorial and all of a sudden I started to change the size of the font for no reason... it's almost just distracting in some cases.
 

 

All in all, it's just a good idea that you jot down on a note somewhere (I.E: Helvetica Size 9) and remember the exact size of your lettering font so that you don't forget it page by page :)

As for what sizes most professionals use... it all depends on the size that you're printing/displaying at.  See what's most comfortable for you.  Generally, comic book sizes for american comics are at size 7-9 depending on the font.

- Use balloons to lead your reader’s eyes around the page.

As you can see from the example above, this is a method to almost force your reader to look through the entire page, and prevent the whole "wall of text" look.

Breaking apart large "Walls of text" is a good thing to do, because it almost gives the page a sense of animation.  It  almost makes it seem like you're drawing invisible comic panels with each new balloon that you make, creating a sense of time elapsing as the reader goes through your page.

- Make space around your text.

As you can see from the example above... the bottom image is just so... cramped! Give your readers a little room to breathe when they read the text so it's easier on the eyes!

- Keep balloons off characters if possible.

I see this done in a lot of manga unfortunately... usually because of lots of text, or whatever the case is, but if you have negative space-- USE IT.  It's the best place to put a balloon. 

That's all the big tips I have for now... but of course, there's a lot more to learn.  Balloon Tales offers a dose of very good information. 

http://www.balloontales.com/tips/

Here's a list from the pros about common mistakes that beginner letterers make.  As you can see, some of these tips might be colliding with others-- but that's only because they're extremely important!

http://twitpic.com/9u6nut
http://twitpic.com/9u6om4

And now... here are some great places to find great fonts.  Try to stick to simple ones, that are clear to read, rather than go crazy over some "cool" font.   Be warned, some of them need payment, but just look further for other fonts that are free :)

http://www.dafont.com/
http://www.blambot.com/

Hope it helped :) take care everyone!