Plan out speechbubble placement!
It's one of those things that, like gutter-width, is such a basic part of comics that it's easy to forget how important it is, but planning out where and why you put your speechbubbles, and paying real attention to how it leads the eye across the page is super-important.
I make sure to draw the little blobs marking where the speechbubbles go right there in the very first thumbnail of a page, so that they're there from the very start. This helps prevent me from making panels too crowded to fit the dialogue on top, and it also helps me keep track of how much text is happening on each page.
Example from the sketchcomics I do for our weekly tabletop campaign:
Those big blobby circles in the thumbnails? Those are the speechbubbles.
Other things to pay attention to re: speechbubbles.
Lead the eye across the page
From the moment we learn to read, our brains are kind of hardwired for reading. If you can read, you can't look at a word and NOT read it as you do. So our brains prioritize text when looking at a comic page. Where and how that text is placed matters a lot! Making sure readers can, with a minimum of effort, understand which speechbubbles to read in which order is important! Avoid crossing balloon-tails, know that we will read the left-most bubble in a panel first, and think that bubbles at the top of panels happen before the ones below, etc.
Use bubbles as visual storytelling elements
This one's a bit more in-depth, but you can use the placement, size and style of a speechbubble to indicate unspoken storytelling things. A regular-sized bubble with very small text in it? Looks like this person is speaking very quietly! A bubble with wobbly, jagged edges? This person is shouting, or their voice is shaking as they speak! A bubble placed in between two characters, like a barrier? These two characters might be emotionally distant from one another, or having a disagreement!
The style and placement of the bubble itself - just like sound-effects and font-style - signal to the reader how they sound through how they look. Keep that in mind when placing and designing them!