The only strategy that I found works for me is to observe and take note of anything that comes up in terms of interests, noticing heavily used items that could use and upgrade or replacement, or even a passing comment about something they'd like to have. Maybe it's just a long, wistful glance through a shop window that you notice. 

It might be a bit late for this year, but the trick is to not start with this a couple of weeks before Christmas, but make it a habit all year round. Whenever you notice something, or an idea crosses your mind, write it down.

Quite a few of those ideas will have become obsolete by the time Christmas or birthdays come up, but you'll still end up with a helpful list to work off.