Quite a few people ask me how they can improve their flexibility and the truth is that there isn't an easy fix. My biggest tip is to let go of your ego. You'll probably want to jump right into a split, but you've got to be honest with yourself, start where you are and consistently work to build your flexibility. Try poses like parsvakonasana, triangle, and reverse triangle.
Another tip is to be really mindful of how you use your breath. It's important to use the lungs to their fullest capacity, as they will help open up the tight muscles. Working on flexibility can be frustrating. You'll be in a pose and it won't hurt but it will just be uncomfortable, and you'll probably want to get out of the pose as soon as you can. Stay present when that happens. This is what they call "the edge." That place just before pain. Breathe full, deep breaths into wherever you're feeling it, and visualize the muscles lengthen.
It's really important to stay present and move with intention. Always, always work within a pain-free range. The last thing you want to do is push yourself too far and then have to sit out for a few weeks until you heal up. Believe me, I learned that the hard way this summer when I had to take about 4 weeks off from all exercise due to an ab injury.
Here is a 15 minute video for building strength and flexibility in your practice. Hope you enjoy.