A discussion on G+’s martial artists forums turned to the difficulties some people have in maintaining their stances in sparring and fighting. Here is the advice I gave to someone which worked very well for me:
One thing that helped me out a lot when I was first starting my kung fu practice (I began with Southern Shaolin) was to spend a lot of time practicing the basic stances (horse-riding stance, bow-arrow stance, false leg, single leg, stream character, and unicorn step) and also moving from stance to stance, especially ensuring proper foot placement before transferring my weight and rotating my knees and waist as the stance requires. It felt very unnatural at first, but now it’s more natural for me to walk the way a “kung fu person” would walk than an “untrained” person would walk (such as throwing one’s weight forward or “falling” forward). I followed my grandmaster’s example of practicing nothing but stances, flexibility (especially of the legs and waist), and footwork for three months before learning any punches, kicks, and other martial movements, and it paid many dividends.
Since you are learning Xingyiquan, I might recommend spending some time holding the Three Body Stance (the main form of stance training in almost all Xingyiquan schools) and practicing moving to and fro in the Three Body Stance (which is very similar to the stream character stance in Shaolin kung fu), occasionally mixing it up with the other basic stances. I also heartily recommend leg and waist stretching exercises so that you can comfortably make the best of your footwork. Over time, the combination of stance holding, increased flexibility, and moving in stances should make you both very solid and very agile. My prediction is that you will see very noticeable results even after one or two months.
You might also ask your Sifu if he/she has any advice to make you both solid and agile. They will be the “last word” on the art that you are learning from them.