Prevent Indenting of first paragraphs

Automatically: no. InDesign does not format any paragraphs automatically (although GREP styles do, for character styles).

You can set the 'with' paragraph as "Next Style" in the 'without' paragraph, and in the 'with' paragraph style, set "Same Style" as its next style. Then select a range of paragraphs and choose "Apply this style, then next" from the Paragraph Style menu.

If you know how to use scripts, well, I could whip up a javascript in minutes that goes over your document and changes any 'with' paragraph without a previous 'with' or 'without' to 'without' and any 'without' not preceeded by 'with' or 'without' to 'with'. With out, I mean without a problem. (Sorry, ran out of my withs.)

Here you go, fresh out of TextPad. Be sure to change the style names in the first 2 lines.

a = "without";
b = "with";if (app.selection.length != 1 || !app.selection[0].hasOwnProperty("baseline"))
{
 alert ("Please click the text cursor in a story before running this script.");
 exit(0);
}
try {
 stylea = app.activeDocument.paragraphStyles.item(a);
} catch (e) { }
if (stylea == null)
{
 alert ("Eh. Style "+a+" doesn't seem to exist.\nChange the name in this script");
 exit(0);
}
try {
 styleb = app.activeDocument.paragraphStyles.item(b);
} catch (e) { }
if (styleb == null)
{
 alert ("Oh. Now style "+b+" doesn't exist either.\nChange the name in this script");
 exit(0);
}
allpars = app.selection[0].parentStory.paragraphs;
for (p=0; p<allpars.length; p++)
{
 if (allpars[p].appliedParagraphStyle == stylea ||
  allpars[p].appliedParagraphStyle == styleb)
 {
  if (p > 0 &&
   (allpars[p-1].appliedParagraphStyle == stylea ||
    allpars[p-1].appliedParagraphStyle == styleb))
   allpars[p].applyParagraphStyle(styleb,false);
  else
   allpars[p].applyParagraphStyle(stylea,false);
 }
}

Not tested on CS3 or newer, but it oughta work. Supposedly, it changes all occurrences of one of the styles to the 'without' style, except when one of the preceding is one of same. Theoretically, it should leave manual formatting alone, but do a "Save" first just in case.

You can also apply the paragraph style to an object style and check the "apply with next style" box, then set it to a hotkey.

Hm. It seems to toggle some paragraphs when run more than once! This version is slightly improved:

a = "without";
b = "with";
if (app.selection.length != 1 || !app.selection[0].hasOwnProperty("baseline"))
{
 alert ("Please click the text cursor in a story before running this script.");
 exit(0);
}
try {
 stylea = app.activeDocument.paragraphStyles.item(a);
} catch (e) { }
if (stylea == null)
{
 alert ("Eh. Style "+a+" doesn't seem to exist.\nChange the name in this script");
 exit(0);
}
try {
 styleb = app.activeDocument.paragraphStyles.item(b);
} catch (e) { }
if (styleb == null)
{
 alert ("Oh. Now style "+b+" doesn't exist either.\nChange the name in this script");
 exit(0);
}
allpars = app.selection[0].parentStory.paragraphs;
for (p=0; p<allpars.length; p++)
{
 if (allpars[p].appliedParagraphStyle == stylea ||
  allpars[p].appliedParagraphStyle == styleb)
 {
  if (allpars[p].appliedParagraphStyle != stylea)
  {
   allpars[p].applyParagraphStyle(stylea,false);
  }
  if (p > 0 &&
   (allpars[p-1].appliedParagraphStyle == stylea ||
   allpars[p-1].appliedParagraphStyle == styleb) &&
   allpars[p].appliedParagraphStyle != styleb)
  {
   allpars[p].applyParagraphStyle(styleb,false);
  }
 }
}

What is the difference between using this script (it works very fast!!!) or using a paragraph style?

I mean in terms of manual operations...

Both oblige to insert the cursor in the without paragraph.

What is de idea behind this? It seems powerful but I do not know how to use it extensively...