Lines are a big part of drawing. When not used for the main part of the picture, they're often overlooked. But that's easy to change!
If there's something unique or surprising about the lines, people will notice. That sense of mystery draws them in. And the visual captures their attention for much longer.
Take a good look at the steam lines coming out of the mug above. There are a lot of ways those lines could have been drawn to give the effect of steam. There are a lot of ways "Merry Christmas!" could have been written, too.
When the words "Merry Christmas!" are written in the steam, those lines suddenly become much more interesting. The words become part of the picture. They're definitely not an after thought!
So use those lines to really unify your visuals!