I was faced with a similar dilemma recently. Prospective employers or recruiters will typically ask for your current salary, which you are not obliged to tell them prior to any salary negotiations.
By revealing your salary you may be going into any future salary negotiations at a disadvantage. They should be hiring you at a level fit for the role, not how much they can get away with. However I know this can be difficult to do, as you may come across as difficult or hiding something.
As such, a few of us might be tempted to take some creative license and bump up our salaries to 'market level'. However I would not recommend this.
1. BEFORE STARTING WORK: At some point they may ask for copies of your pay slips from which they can deduce your actual salary. (once again you're not obliged to provide them this). If this doesn't align with the salary you've verbally told them, then theres a lot of uncomfortable explaining to do.
2. ONCE YOU'VE STARTED WORK: If you work in the UK, you need to hand in your P45 'Details of Employee Leaving Work' (I think the equivalent in the US is the Pink Slip). You'll have difficulty NOT handing this in once you start. These forms typically outline all the pay and tax details for the past financil year which can deduce your actual pay. Once again you'll don't want to be starting off your career at your new employer with black mark.
So basically you can either:
1. Not reveal your salary (your not obliged to)
2. If you do, then tell them the truth. The risk of being caught out is more pain than it's worth.