Before leaving home, I make an itinerary with all travel information, like Pauline mentioned. I email copies to my children, print a copy for myself, and print a copy for my friend and next-door neighbor. When I give him the copy, I include copies of our passports and a copy of the list of credit/bank cards we are carrying, with the numbers to call if the cards are lost or stolen. I have that same information with me, but I feel safer with a back-up at home. Fortunately, I have never had to use it.
When we are staying in a house or apartment for a week or more, I strip out extra cards and our passports and hide them, unless the place has a safe. I usually opt for putting them between pots and pans in the kitchen. While theft out of a dwelling is a possibility, it feels safer than carrying it all around.
Like Cameron, I put cards in an inside zippered pocket of my cross-body bag when traveling or on the street. I never put my bag on a chair or the floor. While dining it stays on my lap, under the napkin.
I program all phone numbers into my phone with a + symbol and country code before departing, including all apartment owners, etc. and any family/friends I might need or want to call while traveling. I also program in the addresses of apartments.
I have a Skype Mobile app on my phone with a small cash balance for "emergencies." Like when I hit the airport in Munich a few years ago and my iPhone wouldn't work. I used the wifi connection and skype mobile to call Verizon and figure it out.
When I check into apartments or houses, I connect to wifi before the person checking me in leaves so that I can have assistance if it doesn't work. I learned that one the hard way.
I take my MacBook Air with me. It really comes in handy for many uses. Last year I developed a problem with one of my eyes just two days into a 6-week stay in Barcelona. I googled "English speaking ophthalmologist Barcelona" and found several--one was a 10-minute walk from my apartment and he saw me later that day.
I make google maps of the places we are going to be for a week or more. Since that shows up on my phone, also, I am able to use wifi to do a map to a restaurant or museum, etc. and then use the map offline to walk there.
Always check to see if there is a metro app in your city. Barcelona, for instance, has a great one. It is useable offline to figure out the best route.