Make sure to trade only what responds quickly and avoid what doesn't. Direct-access issues must have deep representation through ECNs or they will trigger heavy slippage. Discount brokers have order flow arrangements with wholesale market makers that favor certain stocks over others. Some may even fill as fast as ECN matching orders. For example, big Nasdaq market makers spend so much time shaking out scalpers that 1-3 day swing traders can easily fill positions below this radar and turn consistent profits.
Opportunity directly relates to the amount of time set aside for market analysis.
Pick only one or two issues and follow them exclusively if trading time is limited to a couple of hours per week. Several hours per day presents the minimum commitment to successfully swing trade a variety of stocks. Increase the watch list size as dedicated time expands. Success comes more quickly trading 2 or 3 well-analyzed stocks than chasing 20 or 30 poorly understand ones. The ability to watch the markets in real-time determines the appropriate trading strategy.