Selecting Lumber
Start with good quality, straight and level lumber. It's better to custom select it at the lumber yard, as there are differences in the stock. Using 1 x 3 for anything up to the 3-foot or 4-foot range is desirable. For larger pieces, 1 x 4 is better, and for smaller you can use 1 x 2 or even 1 x 1 if the panel is under 1 foot in each direction. It's important to build cradles for larger works, but even for small pieces a cradle can be helpful--the risk of bowing may be less in the small pieces but the cradle provides a good hanging mechanism.