1. Sanding can make all the difference
When painting interior walls you have to start with a smooth surface to end up with a perfectly painted wall. Sanding levels outs spackle or joint-compound patches and flattens ridges around nail holes. Sand the walls with fine-grit sanding paper mounted on a sanding pole. Don't put a lot of pressure on the sanding pole or the head can flip over and damage the wall. Sand trim with a sanding sponge to get into the crevices.
2. Don't buy cheap supplies
Don't cheap out on paint and brushes. Cheap brushes are only going to make the task exponentially harder. Using a cheap brush is going make it look like you smeared paint on the wall with a rake, not to mention the bristles fall out. We recommend buying the Wooster silver tip brushes. These brushes apply paint evenly and won't leave drag marks like the cheap-o's.
You also want to buy the best paint. Cheap paint leads to cheap results and when it comes to wiping off that handprint on the wall you will most likely take the paint off with it. Spending the extra money for high quality paint is going to ensure all your hard work will last for years.
3. Cover up anything you don't want paint on
4. Use a putty knife like the pros
If your furniture is too heavy to move out of the room, move all of it to the center of the room and cover it with plastic sheets that are taped at the bottom. This will protect your furniture from paint drips and splatters.
4. Use a putty knife like the pros
Nothing is more discouraging when you've finished painting than to peel the tape off only to discover the paint bled through. To avoid this apply tape over the area to be protected, then run a putty knife over the top of the tape for a good seal. That'll stop any paint bleeds. Make sure to use the blue painter's tape instead of masking tape. Paint can cause masking tape to buckle or get wavy, which lets paint run underneath it. Painter's tape can be left on for days and still peel off cleanly. And it stops paint bleed without buckling.
5. Paint the trim first
Why paint the trim first? Because it’s easier and faster to tape off the trim than to tape off the walls. And you certainly don’t want to to have to tape them both. When painting the trim don't worry if some of the trim paint gets on the walls, you'll cover it later when painting the wall. Just focus on getting a smooth finish on the wood. Once the trim is completely painted and dry (at least 24 hours), tape it off (see #4. Use a putty knife like the pros) and paint the ceiling, then the walls.
6. Canvas drop cloths are a must
Don't use bedsheets as drop cloths. Thin sheets won't stop splatters and spills from seeping through to your flooring. And while painters plastic can contain spills, the paint stays wet for a long time. That wet paint can (and usually does) find the bottom of your shoes and get tracked throughout the house. Drop cloths absorb splatters minimizing the chance of spreading the spill. For larger spills always make sure you clean them up immediately as it can soak through.
Typically you don't need a giant drop cloth either. Buy a cloth that is just a few feet wide and runs the length of the wall. You can move it easily when you're ready to move to a different area.
7. Box your paint to ensure a consistent color
The "same" color of paint can vary slightly between cans. When using paint from big box stores that difference can be glaringly obvious, especially if you open a new can in the middle of a wall. So to ensure color consistency from start to finish, mix your cans of paint together in a 5-gallon bucket; this is a process known as "boxing". Once boxed together you can simply put in a roller screen and eliminate the use of a paint tray. For smaller areas or ceilings a 5-gallon bucket may be too heavy to handle. For this you can transfer the paint back into the individual containers to make it easier to hold or set on your ladder.
8. Remove light switch and outlet covers
This is a no-brainer. Instead of laboriously taping your outlet covers, grab your screwdriver and take them off. Now you can paint the wall around the outlet and avoid the extra work of scraping paint off the covers later.
9. Avoid runs by "pushing the paint"
When applying paint to corners or around trim it's easy to create runs by applying too much paint and trying to work too quickly. To avoid that, start by brushing out away from the cut-in area. As the brush unloads, move over and slowly fan out the brush bristles and allow the bristles to push the paint against the cut-in area. You may have to do this a couple of times to get complete coverage, but it'll help you avoid excess paint in unwanted areas. REMEMBER cheaper paint leads to cheaper results and cheaper paint is more likely to run and splatter than high quality paint
10. Wrap it up
When you're ready to call it a day, you don't always have to completely clean your roller cover. You can soak your roller in paint and then wrap it in a plastic bag so it's airtight. Only do this if you are planning on resuming the next day. If it's going to be a while before you paint again, you can still put the bag over the roller, then use it to pull the roller off without covering yourself in paint. As for your expensive brush, you can wash that out; assuming you've used a water-based paint. Using your hose find an area that is out of the way and rinse until clean and store it until the next time you need it.