Not that I’m an expert, but since I don’t have major problems using the ‘search’ function, I thought I’d just post my experiences with it here for those who are constantly frustrated by it. I’ll go field-by-field.

Search words: So I usually choose a bunch of words like &quotaris things to do" or &quotaris fun" or "hostels in Barcelona". If you choose one word like &quotaris" or "Barcelona" the search engine gets overwhelmed and then you’ll get the annoying "timeout search expired". If you’re looking for something really specific, like a bar or restaurant, type in all the words. I usually choose search for all words
Search forum Self-explanatory. Just realize that ‘favorite places’ is Most Likely To Fail out of all the searches.
Search in Your best bet is to choose "subject". However, if you are looking for a specific bar or restaurant, you can choose "entire message", but don’t check ‘archives’ – it won’t work, guaranteed. Archives only work if you’re searching by subject, not entire message.
Search by Date I usually choose "Any Date" (your only option if you are checking ‘archives’, BTW), but if you remember the post was recent, you can limit this. You can also go back and limit this if your search doesn’t work with "Any Date".
Search by Member Usually not helpful unless you are looking for something someone said specifically. Leave blank.

Good luck, happy searching. It works quite well. I just did a random search for &quotaris", "subject", "Favorite Places", "Any Date" (no archives) and got many many threads in the past year.