If your snorkel is old, you should get a new one. At a "certain age," snorkels begin to have problems staying erect. Blowing harder through them does not work. There are many temporary ways to treat the problem (snorkel salves, blow up snorkel supporters, even snorkel erector pills you can use--prescription only, of course). At some point, you will just become frustrated with the results and trouble and just decide to get a new one. My advice is to consult with a experienced snorkologist who can recommend the correct one to obtain and help you make contact with a good snorkosurgeon for installation.
I also recommend that you do not waste time and energy just using your hand for support.