iPhone frequently locking up when placed in front pocket

I often place my iPhone in my front trousers pocket. I was told to insure phone faces outward, not towards my body to avoid 'lockup'. This has worked for the past year, until the last few weeks. iPhone is locking up frequently, requiring me to wait up to an hour before I can gain access. Very annoying! What has caused the change, and how do I avoid it from happening?

[Re-Titled by Moderator]

Original Title: iPhone unintended lockup