Lots of good advice in this thread - reading-order of speech bubbles , backing up your work, building a buffer of pages, being patient, etc. So I'm seconding all of those. I'd also like to add a more technical bit of advice.
1.) KEEP TRACK OF YOUR CAMERA. That is, always, ALWAYS remember from which angle you've drawn the scene. If you draw a scene in which two characters are facing each other and Character A is looking to the right at Character B, who is therefore looking to the left, make sure you draw them looking in those directions in following panels, unless they physically change places. If you draw a character facing left in one panel, and then facing right in the next panel, I as a reader am going to think they've turned around. If it turns out they haven't turned around, I'm going to be confused.
The same goes for whatever the character is doing. If they're facing, say, a building on the left that they're headed towards, don't have them heading to the right in the following panel; that's going to confuse me.
For further reading on this, here's a Wikipedia-article to tell you all about this 180-degree rule!
2.) Remember reading order! This goes beyond just speech-bubble placement (though that is super-important too!). As someone who draws in regular, western, left-to-right order, but has read a TON of right-to-left manga, I find this trips me up a lot. If your comic is drawn to be read left-to-right, do your best to keep the action within the panels flowing the same way. Try to have your characters moving to the right, as this encourages the reader's eye to move that way as well. If you have the speech bubbles running left to right, but the action within the panels flowing the opposite way, the eye gets confused
3.) Storyboarding is your friend and your crutch. If you've got an idea of what the next page will look like before you draw it, you not only increase your drawing speed, you are also able to tie pages together better. I storyboard entire chapters at a time, which means that before I sit down and draw the full-sized pages (I do thumbnail-sized storyboards), I can play around with speech bubble-placement, panel arrangements and - in the bigger picture - the overall pacing of the storytelling.