Need help with a couple of photoshop kinks

There are a couple of things I'm really having trouble with and did a little googling and it seems a lot of new photoshop users are having the same problem.

1. Creating selections with perfectly smooth edges - I've tried every selection tool, though the magnetic pen seems to work the best, it's still not quite perfect enough. The gaussian blur/layer mask doesn't give the desired effect and neither does the edge refinement tool. Any ideas?

2. Placing photos and layer fill into existing layers - Isn't there some easy way to do this? Photoshop is programmed to automatically create a new layer so you can't fill in a shape.

Thanks