For the buttons, there are two items of importance: making sure you have a thread that is strong enough to hold the button on, otherwise it will sag. I frequently use embroidery floss or quilting thread to ensure security. For thread, making multiple passes through the button in an X motion is important. If the button is subject to a lot of strain as you move around, make a little bit of slack in the thread holding the button - it doesn't have to be super tight.
The second is making sure you knot the thread on both ends. Cut a thread that is the length of your forearm and pass it through the needle so the thread is even and make a knot with both of the threads. Knot multiple times - about 3 or 4. This will make it so that you are sewing with a double thread, but it is worth it for security. When you are done sewing the buttonhole, pass the needle under your last stitch, then pass the needle through the loop that makes, then pass the needle back around through the loop again and pull it tight to form a knot. When the knot is formed, cut the thread, and then you can remove the remaining thread from the needle.
Threadbare clothes should be thrown out, but if you just need a seam, I recommend learning a backstitch. Using the double thread I mentioned, pierce with the needle and pull the thread tight against the stopper knot. Move the needle about a half a millimeter on the side the thread emerges from and pierce through, pull tight so the thread is flush with the fabric, and then pierce back in next to the stopper knot, pull that tight. Move the needle a half a millimeter past the first stitch, pass through, pull tight, then move the needle back to next to the front edge of the previous stitch, pass through, pull tight. If you're confused, get a video specifically on back stitches.
I recommend this stitch because every stitch is a knot of sorts. So if you tie a knot wrong at the end of your seam or one that isn't perfect, your stitches will remain secure. A standard running stitch is only as good as its knots and you'll be mending constantly. A backstitch will last for years - I use them for handbag seams and they never come out. Tough as nails.