I'm writing a book on Google using OpenOffice Writer (latest stable
version). I want all my paragraphs to have an indented first line of
0,60cm, except when they are directly following a headline (I believe
that's common style). What I managed to do already is choose
"Paragraph (X)" from the style select box... but then I have to click
Format -> Paragraph -> 0,60cm everytime I'm creating a new paragraph
on a new page (it will only "remember" the setting when I type
multiple paragraphs in continuation).

My question: How do I force a paragraph default style in OpenOffice
Writer for this whole document, and also make sure I can change this
value at the end in case I want to optimize something?

First, create a style with your indent set properly. 
  In Writer, Format->Stylist. Then right click on your body text style
and do "New". Give your style a name. Your normal body text style
should be in selected in the "Linked With" popup in the Organizer tab
in that dialog box. Also make sure that the "Next Style" popup has
this style selected.

Second, modify your body text style to use, as its "Next Style", the
style you created in step 1.
  Right click on your normal (non-indented) body text style in the
Stlist. Select Modify from the menu. In the Organize tab select the
style you created in step 1 as the "Next Style"

Thirdly, you want to modify the Heading style to use the style you
modified in step 2 as (Headings) next style. Instructions are the same
as for step 2, except select (your normal, non-indented body text
style) as the Next Style.

This set up will allow you to create a headline paragraph, press
return, and Writer will automatically create a non-indented paragraph
for you. Press return again, and all of your paragraphs from then on
will be indented.

Thanks a lot Ryan, I'll try that!