Clean as You Go
Have paper towels and damp rags handy to grab any drips or spills as you paint. Trust me, drips and spills happen to the best of us. Paint comes off of slick surfaces like tile and glass windows when dry if you use straight razor blades. So if you drip on a slick surface, continue painting, let the paint stiffen and scrape later.
Also, if you're working over a rug, have a rug-cleaning product handy. Even if you have a drop cloth down, paint may seep through and can stain carpet!