In general, federal policy has a strategic effect on economics. Taxation, regulation, the federal budget, and expectations of future decisions all factor in. Congress traditionally and by law controls those things.
War has a relatively immediate effect. Now, after Libya, presidents can wage it on their own without the approval of congress, they have the power to stimulate the economy through emergency military spending.
Regulatory movements have an intermediate effect. This administration is expanding the power of the executive branch to pursue basic policy goals without enabling legislation simply by executive order and departmental directive. Given the inertia of the massive federal bureaucracy, it takes time to change course, and so these effects are not immediate. Like most regulatory effects, they tend to be negative simply because they by definition encroach on what before were fields of free enterprise.
Other than that, presidents have little ability to influence the economy. The answer is that while their power to do that is growing, they still lack the tools to grasp it firmly.