Avoid sharp edges on your veneer preps, we need to have rounded corners on our veneer preps.
Prevent any undercuts (especially on the slice prep or deep elbow preps). View the preps from various angles to make the sure veneer preps have a taper.
Make sure there is sufficient reduction. I prefer to use a reduction guide to make sure I have equal reduction. I use a facial reduction guide and an incisal reduction guide.
Darker preps or a crown prep next to a veneer prep. The veneer prep may need more facial reduction if next to a crown prep. A darker prep/ darker tooth may also require more reduction/ more preparation to help balance out the color/ shade.
Do not have a finish line at the lingual contact point. It should either be above or below the contact point.
Use the slice prep to close spaces and black triangles. Get a wax up to help have the end in mind and to plan your smile makeover cases.
Use incisal and facial reduction guides to verify reduction. Key to facial reduction is to have 3 plane reduction, focusing on the incisal-facial third. This can help prevent show through and thin areas of the veneers.