Here's a shirt I got last year at Target in Atlanta. Love the colour, love the cotton... do not love the fact that it was the last one on the rack and therefore two sizes too large. I had it in my gigantic stash o crap to sell at a carboot sale taking place at an undisclosed time in the future, but then I had an idea to recreate this little tee tree shirt into a wee summer top.
First I cut it straight across from armpit to armpit. With the remaining tube top I did a zigzag stitch to finish the resulting raw edge. By unpicking the collar salvaged from the other smaller piece, I had what would become the straps. I just sewed a straight seam along the length of the collar and used a safety pin to turn it inside out to create a thin tube of fabric. Once I had one long strap, I cut it in half, knotting one end of each of the resulting short lengths.
But back to the top itself: I pressed the new edge under, then did a straight seam, leaving a tiny hole rather than sealing it all up. Into the hole I threaded some elastic, again using a safety pin to guide it. Once I had the two ends of the elastic poking out of the hole I sewed them together and sealed up the hole with a seam that followed the existing seam. I sewed one end of each strap to the top edge of the front of the shirt, equally spaced from the side seams of the tube top. The other ends could be tied behind my neck. Don't take my word for it though; you can see a selection of how-to pics on Flickr.
Now I'm ready for summer. If only if it would stop hailing outside.