Sand With the Grain
Sand with the grain when hand sanding or using a belt sander. Scratches are hard to see when they run parallel to the grain. But even the lightest scratches across the grain are obvious, especially after staining.
Inspect Before You Stain
Turn out the lights and shine light at a low angle across the wood to reveal imperfections. Flag the problem areas with masking tape and sand them out.
Consider a Wood Conditioner
Some woods absorb stain unevenly, which causes dark blotches to appear. Birch, maple, pine and cherry can all play this ugly trick on you. It's hard to eliminate this effect, but you can limit it by applying a wood conditioner before staining. Conditioner also prevents wood's end grain from absorbing more stain than the face grain. Get a quart at a home center or paint store.