Paint a Faux Sky
Begin by preparing the wall with a primer if necessary and a basecoat of white latex paint. Allow to dry. Then, with a sponge, use a circular motion to apply blue latex paint over the wall surface, coating it completely, but leaving some variation in paint thickness. Repeat this step after the first coat of blue has dried.
If desired, use a pencil to draw in rough cloud shapes. Don't press too hard with the pencil - use just enough pressure so that you can see where your clouds will go.
Use a 1:1 mixture of white latex paint and wallpaper paste for the clouds. Use a stiff brush to stipple (a quick, dabbing motion with the bristles of the brush perpendicular to the wall) paint onto the surface. When painting the clouds, always work from the top of the cloud shape downward without reloading the brush. This will produce a more realistic effect, as the clouds will appear more transparent towards the bottom where the white paint is not as thick.
When the clouds have dried completely, go over them again using the same method as before. You may also want to create fluffier clouds by building them out - make mini-clouds within the large clouds to create depth and form.
It's up to you how many clouds you want, or how fluffy and distinct you want them to be. Go over the clouds with white paint as many times as desired. When finished, you'll have a blue sky to look at even when it's raining.