The trick to getting green beans which are crisp on the outside and tender on the inside is to boil them fast and hot, then stop the cook by plunging them in cold water. Don't be stingy with the water you boil them in–a little extra water helps the pot stay hot after the addition of the beans.
Usually the beans will take 8 minutes to become tender-crisp. They can then sit a bit until you're ready to serve.
Once you're ready, put them back in the pan and reheat gently to drive the water off and bring to temperature. Then you can finish them with a little lemon juice or a little butter with black pepper. If you add butter while the beans are still wet, the butter will slide off and you'll get an oily mix at the bottom of the pan. It's not a disaster, just a waste of delicious butter.
Green beans are really delicious when mixed with mushrooms fried in butter. Fry the mushrooms seperately and combine at the end.
Mushrooms fried with garlic and butter are delicious, but if you try to do a big batch they can be a little annoying. They start by soaking up a ton of butter, then they turn around and release a ton of liquid. It's not so noticable in a small batch because the water cooks off quickly.
If you want to do a large-ish batch and don't want to work over the stove for ages doing a few at a time, one trick is to dry-fry them by not adding any butter or oil to the pan when you start out. This encourages the water to cook off faster. Stir frequently. Then when the mushrooms are done to the level that you want, finish with a little garlic and butter. The pan wil have sticky mushroom bits baked on, but the mushrooms will be delicious.