I bought Shift from Bittrex for long term hold. Where can I securely keep my coins?

i installed a virtual machine on my private network with shift installed. i only login there. otherwise you could use the official wallet https://wallet.shiftnrg.org and create and wallet there. if you will have stored some shift in your account, don't forget to create a 2nd passphrase for your account.
it's more secure than leave your amount on bittrex and you can vote some delegates and delegate pools, from where you can earn some shift every day. the amount you get depends on how much shift you have. the more shift you have, the more powerful is your vote.
for example, if you have ~100k you will get ~1 shift per day from a pool.

best regards