I just plant the depth specified on the package, and fertilizer. Mine always do well. The problem I have with Dahlias is wintering over the bulbs.

Keep in mind that dahlias need everything tomatoes need to flourish:

-Planted after frost in your area (often cited as Memorial's Day in the Northern US regions)
-Once sprouted/rooted, lots of water in well-draining soil
-Slow-release fertilizer to avoid delivering too much or not enough nutrients
-A support system to keep the top-heavy plants from collapsing onto the ground

A great resource for growing dahlias can be found here:
"Growing Dahlias FAQ"

Hope this helps, and may you fall in love with the wonderful world of dahlias!