If I am wearing an expensive watch, what should I do not to get in trouble at customs? Should I declare it when I enter a country?
I will travel from Germany to Russia.
The Customs rules for most countries are usually "clear enough" re what needs to be declared and what does not need to be.
Read them very carefully. Ask if uncertain. I have seen a number of people caught out by not properly doing so. (Usually with respect to agricultural imports).
It seldom hurts to err on the safe side and to declare items whose status is unclear. There may be a requirement to pay a bond in some countries for items above a certain value, to ensure that they are re-exported (and I understand that this may in fact be the case with some items in Russia) - in such cases declaring an item unnecessarily may involve more paperwork and temporary loss of the bond BUT in situations where this applies, failure to do so may result in loss of the item or worse when you leave.
However, for me the biggest concern with a valuable watch would be whether it was an increased liability to wear it in public in countries where security levels are lower than in Germany. The actual cost of the watch can be mostly covered by insurance, but if you attract people who consider that they have more "right" to it than you do, the trauma from the resulting transaction may well exceed the gains from wearing it. (I have a friend who was relieved of his watch and misc other valuables at shotgun point in PNG. He was surprised and pleased to find himself alive at the end. The value of the goods lost was immaterial.)
I take photos of all the 'gear' that I carry before leaving and often also occasionally on trip. While this is not official proof of what you had when it does form a reasonably convincing backup to documentation.
I have travelled reasonably intensively in Asia in recent years. I visually tend look like I'd benefit from a good meal and some new clothes (apart from the ever present somewhat too flashy DSLR camera). . So far I've had no aggravation anywhere :-).
Really, if you are concerned about, leave it at home. Buy a $5 watch if you feel the necessity to have a watch. This is one worry which is so easy to avoid.
Now if you want to travel with it, you can. Sometime between departure and arrival you will get a Customs Declaration Form and the rules are usually found on the back. Any embassy for the destination country should be able to provide you the form in advance.
When there is a personal limit as to good you can bring in, it will be stated and you will have to fill the declaration statement should your watch exceed that amount. Some countries have small limits (under $1000) and some large ones ($10000).
Exceeding the limit usually means filling out an additional declaration for the item with its serial number and value. You may be asked to deposit import duties on arrival which you get back on departure when you show that the item is still with you.
You're more likely to get in trouble going back home with it from a country where those things are a lot cheaper to buy and/or there's a lot of counterfeits going around.
Take your original receipt (or at the very least a copy showing it's made out to your name) from a dealer in your own country you can present to customs if challenged.
I am also a fan of expensive and high-quality watches. and as rightly noted above. Just point them in the customs declaration as your personal property! but in general, as shown, for such things as watches, Russian customs and border guards virtually ignored.