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What's your application domain? It depends.
Since you used the word "Agile", I'm guessing it's a web app. I have a nice easy answer for you.
Go buy a copy of Burp Suite (it's the #1 Google result for "burp" --- a sure endorsement!); it'll cost you 99EU, or ~$180USD, or $98 Obama Dollars if you wait until November.
Burp works as a web proxy. You browse through your web app using Firefox or IE or whatever, and it collects all the hits you generate. These hits get fed to a feature called "Intruder", which is a web fuzzer. Intruder will figure out all the parameters you provide to each one of your query handlers. It will then try crazy values for each parameter, including SQL, filesystem, and HTML metacharacters. On a typical complex form post, this is going to generate about 1500 hits, which you'll look through to identify scary --- or, more importantly in an Agile context, new --- error responses.
Fuzzing every query handler in your web app at each release iteration is the #1 thing you can do to improve application security without instituting a formal "SDLC" and adding headcount. Beyond that, review your code for the major web app security hot spots:
Use only parameterized prepared SQL statements; don't ever simply concatenate strings and feed them to your database handle.
Filter all inputs to a white list of known good characters (alnum, basic punctuation), and, more importantly, output filter data from your query results to "neutralize" HTML metacharacters to HTML entities (quot, lt, gt, etc).
Use long random hard-to-guess identifiers anywhere you're currently using simple integer row IDs in query parameters, and make sure user X can't see user Y's data just by guessing those identifiers.
Test every query handler in your application to ensure that they function only when a valid, logged-on session cookie is presented.
Turn on the XSRF protection i