There are different apps that work on android if you have an android phone like Trip Log that track your mileage and separates personal from business miles. For a few dollars a year, you can back it up in cloud or just buy a mileage book from staples for a few dollars. I prefer the app because sometimes I forget to write down my milrage and I like to track my expenses in real time. As for posting things in your car, I would be careful because it could hurt ratings or chances for tips.
I use the Sherpa share app on the phone. It tracks each trip and you can code whether personal or work. I also keep an Excel spreadsheet of all my rides. All you need to do is when starting out record the mileage and then at the end of the day record the ending mileage and put into a spreadsheet and calculate all for you. I am an accountant by trade so I am used to keeping track of different files and numbers. It is all a matter of preference. Staples or any other store similar will have mileage log books, just check the business section and you can get all sorts of fun things that may help. I agree with CROWBOY that you shouldn't put any posters or placards as that will just lead to low ratings or disgruntled passengers, which you don't want to deal with. 2/3 of my trips are less than 20 minutes and less than 10 miles, so I just get them in get them where they are going and get them out. No need to piss off passengers.
I agree, there are times where Sherpa Share doesn't track mileage, but that has been on rare occasions and usually when there is a lot of traffic and I go into New York City. Other than that it is good. Like all technology there are bugs that we deal with. A missed trip here or there won't be the end all, and at the end of the year you can adjust for it on your taxes, it will be an insignificant adjustment when all is said and done.