A major part of the travel preparation is to obtain all required visas. Unfortunately nobody tells you which visas you need to obtain before departure, which you can obtain while travelling and which you can receive at the border. The main reason is that visa requirements differ per nationality and that they are also subject to change. My recommendation is that you check-out the website of a local visa service such as CIBT. I also recommend that you double-check again at another source in case you plan to obtain your visa while travelling or at the border. I nearly missed to obtain the Ethiopian visa before departure because I read that it can be obtained at the border. Later on I figured out that border in this case only means at Addis Ababa airport but that it is not possible to obtain a visa at any of the land borders. So these details are important to notice.
You also need to have an eye on the documents which you might need to hand-in to support your visa application. Here are some examples: For the Iran visa application you will need a reference number which your contact person in Iran obtains for you at the Iranian Ministry of Foreign Affairs. Since a few weeks you will also need to confirm that you will purchase a health and accident insurance from an Iranian insurance company upon entry. As a female traveler you will need a passport size photo on which you wear a headscarf completely covering your hair. For the Turkmenistan visa you will need an invitation letter from a local tourist organization and the Uzbekistan visa application requires a confirmation letter from your employer that you are not travelling for work to Uzbekistan. Luckily this letter is not required for German citizens.
It also needs to be considered that many visas expire after a certain amount of time. For instance the Sudanese visa is usually only valid for one month after issue. When you are a long-term traveler who has left his home country months ago this means that you need to apply for the visa while travelling. The application also requires different amounts of time at different locations. While I needed four weeks to apply for the Sudanese visa in Germany, I know US travelers who applied for their Sudanese visa in April and who still did not receive it when leaving for their journey in October. About half of our group was actually able to obtain their Sudanese visa at their home country while the others will try it on the road in Egypt. It is said that Sudanese visas in Cairo or Aswan might only take 1-3 days.
As a long-term traveler you also want to consider applying for a second passport. I did it when I noticed that I otherwise need to return to Germany for several weeks just to apply for visas. So I’m travelling on one passport while the other one stays at home with my friend Michael who will apply for a number of my visas in Germany. However, you cannot simply obtain a second passport in Germany but need to have a valid reason which you have to hand-in in writing.