OK, enough suffering of reading all the comments of you guys. Read the below steps very carefully and think before you start screaming "it doesn't work", well guess what.. it does!
I have iTunes 11.1.3.8.
1. Open up iTunes and stay in the iTunes Library.
2. Select "Music" and go to the "Playlists" tab. (Screenshot: http://imageshack.com/a/img28/6189/glfo.jpg)
3. In the bottom left corner there is a "+". Click on it and select "New Playlist". (Screenshot: http://imageshack.com/a/img202/5175/uoqw.jpg)
4. Name your playlist however you'd like and click "Done". (Screenshot: http://imageshack.com/a/img13/1294/1dm0.jpg)
5. Drag and drop the songs you want on your iPhone into this playlist. (Screenshot: http://imageshack.com/a/img30/2729/thls.jpg)
6. Go to your iPhone, and select the tab "On This iPhone". You do not have to check "Manually manage music and videos" (see screenshot). However, I do have this checked on my main computer to which my iPhone is synced. (Screenshots: http://imageshack.com/a/img12/3931/f9ul.jpg, http://imageshack.com/a/img6/3020/ynvc.jpg)
7. On the bottom of the list you have "Autofill From: (dropdown)". Select the playlist where you added all the music you want to have on your iPhone. (Screenshot: http://imageshack.com/a/img703/4499/7jzr.jpg)
8. Click on "Settings" and uncheck all the boxes (especially the top one!). Then click "OK".(Screenshot: http://imageshack.com/a/img716/5706/s6tu.jpg).
9. Now click "Autofill" and iTunes should add the songs to your iPhone without deleting anything.
Once again, read carefully before you start asking questions!
(thanks man u really help me a lot...now i can manage my songs w/out erasing all my songs... this is the best way! so read carefully)