I think it's best to keep stuff in your front pockets as it's easier to keep your arms or hands in the way. It's also easier to feel if an item is still in your front pocket. However, make sure not to noticeably feel your pocket with your hand as this tips off thieves to which pocket contains your valuables. If you happen to pat your pocket, pat your empty pockets as well to throw them off.
Additionally, you can buy an iphone case with a strap. Even if you don't use the actual strap, you can attach a chain or a keychain to tether the case to either your belt, your pants, or something else. This is similar to the old wallet chain idea.
You can get a retractable keychain but this might be easier to circumvent because there's less tension. However, it's many times more secure than nothing and also more convenient when you want to use your phone. Just make sure to securely attach the reel so that it's not easy to unclip.
Also, I would recommend replacing any clip like the one in the following picture with a keyring or something more secure.
Finally, I've seen retractable iphone cases but it might be better and/or cheaper just to make your own. It's probably best shop around to get a better idea.
Your friends might still try to steal your phone for fun but most thieves don't like to do extra work which is why they steal in the first place.