Some job seekers are so anxious to work that they will accept any dollar amount thrown at them without thinking. This is understandable given the current economic climate, but the desire to have a job doesn't mean you have to settle. You need to make sure you do the proper research into typical salary data for positions to ensure you are being offered a fair deal.
A good tactic to practice in any salary negotiation is to think a little before saying anything. This will indicate to the hiring manager that you aren't entirely sold. If you're lucky, he will up the offer without you having to do anything.
More than likely, however, you will be in for a more protracted negotiation, but your initial hesitation should be enough to show that you are not overwhelmed by the offer. This will give you the opportunity to share the number you had in mind. Assuming you did the proper research, the salary you mention should be in line with industry standards, but that doesn't mean the employer will bend immediately. In fact, there's a good chance they will say "take it or leave it" if they don't have the budget to offer you more.
If the job is too good to pass up, you could always ask if there will be a chance to have a salary review. If the answer to that question is yes, you should probably take the offer. Otherwise, you should consider looking for another job that will pay you the salary you had in mind.