Anyone who has tried to build up their savings account in recent years has likely been disappointed with their earnings. This is in part because record-low interest rates have made it difficult to earn enough to keep up with inflation, and in part because today's fragile economy has made building principal a challenge for many. But those who have been paying attention haven't let weak interest rates prevent their savings from growing. Instead of relying on interest accrued the traditional way, these savvy consumers have turned to alternative means of building savings -- such as bank sign-up bonuses. Incentives of this sort aren't always worth the hassle, however. Here's how bank sign-up bonuses work and what you should look for before chasing one.