How do i tighten up loose skin after weight loss?

In january I weighed 190 lbs. By march I was 170s. Then by may I was 150s. It is now july and I remain the same. I have alot of loose skin on my biceps and stomach. Is there a way to help tighten it up or do I just have to wait awhile? I thought maybe there was a natural supplement that would improve my skin elasticity? I do lots of crunches and have a 6 pack but it is not visible…