RWD major challenge these days is testing your content on all Mobile browsers. The desktop side of RWD that will fit and scale inside a desktop browser is relatively easy as there are no more that 6-7 different types of browsers.
On the mobile side, its entirely a different story as there are 40 + mobile browsers out there in the wild (default + installable). What you need is a “device” plan of attack. Know which device classes you intend to support before you start to code and create content. Have both your lowest common denominator devices and your high-end device designs before you begin to code. Try to visualize a way to create both versions from one code base and Test on different mobile devices from the beginning to the end to ensure that your incremental work will display correctly in the intended devices. Its good to have a small design lab where you can set up all these mobile devices and test your content. Designing and supporting multiple mobile displays takes time and patience.