Updated: May 4, 2020
There's nothing worse than a patchy paint job. To ensure a nice smooth coating, take the time to prep your walls. Any cracked or peeling paint should be scraped or sanded. Any holes whould be patched with wall putty and sand the surface smooth. Furthermore, oily and dusty patches can prevent paint from sticking to the wall. Before painting, wash your walls and let them completely dry before taking a brush or roller to the wall.
TAPE! You might see these tips and tricks on pinterest that help you keep a straight edge in corners and along trim, but nothing has been more consistent in keeping a neat edge over the years than painters tape. Take your time taping. And after your paint is dried, use the corner of your multitool, or a box cutter, to break the bond between the paint and tape, before removing the tape. Do not leave your tape up longer than 24 hours. Always use a good quality painter's tape. And don't tape outlet covers. Remove oulet covers and tape over the face of your outlets.
Paint can vary in shades slightly between cans. To prevent a break in shades on your walls, box your paint. The process of boxing paint is to mix all cans of the same shade into a larger bucket to maintain consistency in color on the wall.
Don't chince on materials. Painting is a chore that most don't like doing more than they have to. Make sure you buy quality paint products. It's best to buy the paint with the primer. Buy the good paint brush for edging. And prevent getting roller knap in your painted walls, but purchasing a quality paint roller.
Let the roller do the work! When you have a quality paint, it will flow easily onto your walls, so you won't need to use as much pressure as you might have had to use in the past. Using a roller extension will ensure you're not stretching too far and applying more pressure than necessary.
Cut in your lines around the trim, ceilings and borders of your walls. Then start from the top of the wall and work your way down. This will help you see any paint discrepancies, like drips, to remedy as you're working through the wall. Also, once part of the wall is starting to dry, don't paint over it again until it's completely dry. Painting over a partially dried wall will cause marks in the paint, and could even remove paint.
Ultimately, you should take your time and do a proper job. This is a feature in your home you have to live with for years to come. The last thing you want to see, after all the work of painting, is flaws in your work.