How can I keep my english in a good level if I rarely use it in my job, for example..?
Talking to native speakers is the best way to keep your knowledge of a language up to date and even to develop further. If you are uncomfortable with talking to a native speaker at first, you can start by talking to yourself, and find a native speaker friend later. Watching films (first without subtitles, then with English subtitles), videos, listening to songs and reading also helps.
Time permiting, you'd better learn from English materials such as newspapers, TED, English study forum and so on. Besides, having several language partners to practice with is always the best way to keep your oral English in a good level.
yeah, try to read in English and do it in loud voice. On other hands you can try to find people to have live conversation to practice. If you have any hobbie, try to do it in English. For example, if you like watching movies or tv shows, then try to do it in English without subtitles at all.
Good question, I think we should find someone and talk with them day by day in English about anything we like to practice speaking. Moreover, we can read an interesting book in English or an article, discussions on Italki and write comments below. That makes sure we don't forget reading and writing. Finally, listening any songs in English that you like.
keep talking to someone who speaks english