Browsers have fast become one of the biggest memory hogs on modern PCs. The reality is the web is becoming more media-rich and as a result, browser memory usage is increasing. Chrome, as lightweight as it purports to be, is no exception. While there isn't a great deal you can do to optimise it, by understanding what's going on under the hood, you can minimise its footprint.
Martin Brinkmann over at gHacks has put together a short guide on navigating Chrome's various memory tools, such as the Task Manager and the internal memory breakdown page. He also recommends several extensions -- One Tab, The Great Suspender, Tab Hibernation and Foo-Tab, to keep your tabs in check.
That said, if you're stingy on the number of tabs you do (and leave) open, these extensions probably won't help much.
What will find is that your plugins -- Shockwave (Flash) being the main offender -- will get bigger over time. You can monitor their usage using Chrome's Task Manager (Shift+Esc) and when you notice one is taking up too much memory, you can simply terminate it.
Keep in mind this will kill any media using that plugin (so a cached YouTube video in an open tab will need to be refreshed), but it's a good way of keeping the browser's memory consumption streamlined, without closing the whole thing down.