How do I pronounce the th after an s or f 

1) I was there
2) If this ...

I always have a little break before the th and it doesn´t sound very good.

Can you give me an advice how to pronounce it better?

Thank you Emotion: smile

1) was there --> You could try making the 'th' sound a bit more like a /z/. Then the tongue won't necessarily touch the back of the upper teeth anymore, but that's quite acceptable in my opinion.

Note: the 's' in 'wa s ' is actually a 'z' sound, similar to 'zz' in 'bu zz '. So there's a clear distinction there.

2) If this --> probably a matter of careful pronunciation.

- articulate your 'th' very gently; it's a very subtle sound, and it's certainly not a /t/ or /d/.
- place the tip of your tongue just behind your upper front teeth and keep the rest of your tongue relatively stable and anchored to the upper href="http://en.wikipedia.org/wiki/Molar (tooth)" title="molars" target="_blank">molars.
- try not to place the tip of your tongue beyond your front teeth (except in very slow, careful speech).
- recording your own speech and reviewing it will probably help you to get the sound right.

Last but not least, native-speakers of English are lazy speakers. The higher the rate at which they speak, the more pronunciation rules they bend and break, so to speak. It takes some effort to get it right, but eventually it'll pay off.
This is tricky because the letter "s" takes two sounds ~s and ~z and the letter string "th" takes two sounds as well, voiced as in "that" (~that) and unvoiced as in "thin" (~thhin).

About 74% of words ending in "s" in typical text are pronounced ~z , including popular words such as "is, was, has, as, ~iz, ~wuz, ~haz, ~az, (note ~ means written phonetically in truespel). These words are repeated often, and in this analysis the repetitions are counted.

You can check pronunciation using the free truespel text converter which sorts all these sounds out.

tz, creator of truespel