Consciously try to stay loose and relaxed, from your grip to your elbows and biceps/triceps, into your shouilders and neck. I catch myself tensing and have to remind myself to get loose again multiple times during a long ride. The more I increase my awareness of muscle tension the longer I seem to go without tensing up again. It takes practice. It's a combination of getting looser in the shoulder-hand range and simultaneously remembering to focus on using your core muscles in opposition. It's hard (at least for me) but it is a behavior that can be learned.