Scree slopes are composed of tiny fragments of broken rock that may be inches or feet deep. Much like sand, scree presents an unstable surface that shifts with every step you take; the adage "Two steps forward, one step back" definitely applies here. Typical scree consists of rocks that range from quarter- to fist-sized, although talus--large fields or slopes covered with boulders--is actually just large scree.
Sandals are one of the most uncomfortable footwear choices you could make on a scree slope. The open toes will let in small rocks and dust that will wedge between your toes and underneath your feet. Avoid soft mesh or "water" shoes also and protect your toes by sticking to sturdy, closed-toe sneakers or, even better, hiking boots. The extra support of sturdier footwear will also help you keep from twisting your ankles as you walk on the unstable scree surface.
If you're hiking uphill on scree, kick the toes of your boots into the slope with every step. The force of your kick will help compact the scree underneath the toe of your boot, making a more stable surface for your step. The scree may still slide, but it won't move as much as it would have if you didn't kick a step into it.
Follow Established Trails
Look for packed-down indentations in the scree slope. These are trails where others have compressed the scree with their passage. If you follow in their footsteps, you'll enjoy the benefits of more stable footing as well.
"Screeing" is basically creating a controlled landslide in the scree and then riding it down. Just start taking large, fast steps down the scree slope--the impact of each landing will start the scree moving and soon you'll be able to just stand solid on both feet and ride the slide down. Screeing works only if the scree on the slope is thick enough--several inches at least--and should be attempted only on an open slope with few large obstacles, such as boulders, below--where you might slide into them--and above, where they might roll down onto you as the scree slides. Limit your screeing to slopes without precipitous edges and places where you can grab a solid tree off to the edge of the scree, or otherwise exit the scree slide in a hurry if you can no longer control your descent.