I use the professional version of TaxAct, which serves my needs quite well.
It has an excellent interface, supports the state tax returns for every state that has a state income tax, is fairly priced (for BOTH the software and the efile option), plus they have software packages for corporate and partnership returns.
It is NOT as sophisticated as some of the more expensive tax software packages (which can cost as much as $4,000), but I have not had a need for that level of sophistication.
How you market depends on your target audience.
If you are going to compete with H&R Block, Jackson Hewitt and Liberty for the average taxpayer, you need to advertise in your local paper, put out flyers, take out ads in your church bulletin, etc.
If you targeting a niche market, or only want to work high-end returns (small business, corporate, partnerships), you need to specifically target that niche.
You should definitely join the National Association of Tax Professionals (
National Association of Tax Professionals), because they offer, research, support and training, plus they publish a monthly newsletter and a quarterly magazine which offers guidance on how to run and market your practice.