Hi Jelly,
I'm on cycle 15 trying to conceive my first, so can't say "it's worked for me" - but the best thing I've found is tracking my body temperature.
I switch off the alarm in the morning, reach over to snooze the alarm and pick up my digital thermometer, then stick it in my mouth till it beeps. Read temperature, write it down, then start my day (or snooze for another few minutes).
Just doing this every day can tell you when/if you ovulate, and when your period is due to start. It can also give you a good indication whether you are pregnant if you have a high temperature for more than 18 days (good way to hold yourself off of taking a test early).
I have irregular cycles, though I think it's within 'normal' ranges, but I have found my BBT to be the best way to track everything about my cycles - my temperature stays fairly constant around 36.5 before ovulation, then goes up to about 36.7 post ovulation. The morning my period starts, by temperature drops down to about 36.4. Apart from the disappointment of knowing I'm not pregnant, I really like knowing the exact day my period will start. The 18 day rule has also saved me a fortune in pregnancy tests.
The downside of this method is it only shows you when you have already ovulated - and at that point it's too late to do anything about it! If you have regular cycles though, then it should be possible to predict your fertile time for the following months. It can also give you peace of mind - the morning you get your high temperature you ask yourself 'did we do it in the last 4 days?' if yes, you feel good, if not - well, you know not to get your hopes up.
Hope this helps a little.