I get mine laundered and ironed at the dry cleaners round the corner from work. It costs £8 for five shirts which I reckon is about as cheap as doing it yourself at home, especially when you factor in all the hassle of having to take them to and fro, doing all the washing and ironing at the weekends etc. when you should be relaxing.
If this is not an option, I used to take mine in rolled up in my panniers. You can make a nice little roll out of a shirt, pants and socks. When you unroll it, even if weeks later (I kept mine in a drawer in my desk), it will still look fine. To do this you need to learn the proper way of folding shirts, i.e. like they do in the shops, folding the sleeves in etc.