I share your difficulty and this year have decided on (don't laugh) an ice-ax and crampons.... I do at least know this is what he wants!
I have the top gear tickets purchased for his birthday, since that's actually the day they are here so it makes more sense that way!
Normally hubby hates what I buy, but last year I scored a success with a new mobile phone for xmas and a new mac pc for his birthday (that was abit pricey though, but he needed a new pc for his self-employed work so it was worth it).
Like you, clothes fall flat every time so I have given up!
I quite like executive luggage type gifts - a nice travel wallet, briefcase etc, they've gone down ok in the past - the sort of think you tend not to spend your own money on because there are cheaper option that "are ok" but the expensive ones make better gifts!
Other gifts that have worked in the past included nice cufflinks (but only works if he wears that type of shirt); a hiking watch with altimeter (which he still uses 10 years on, so that really was a good idea); a watch (better for birthdays though)...
Finally, one other option which can also be great fun for Christmas is a collection of little things, all wrapped up etc, so it's like a "lucky dip" - especially if it is little things with some meaning or fun stuff, like, his favourite food that's hard to get in HK, or a book or a CD etc ... my sister did this last year at Christmas and it was really good. She purchased and wrapped up (for hubby and I together) a box of food items like Bisto gravy and stuffing mix and stuff, because she knows that I don't cook enough and that hubby loves a beef roast .... so opening this in front of the family was absolutely hilarious (it had the added bonus of being really cheap for my bankrupt sister, who says you need to spend a fortune to give good gifts?).