The best way to tackle it if you don't have irregular/very long cycles is to chart for a month or 2.
This allows you to note down your cervical position-it will go higher and softer as you approach O.
It allows you to note your cervical mucous- this will become more abundant and have the texture of egg white, all stringy and stretchy.
It allows you to note any cramping which can indicates imminent or actual release of the egg.
AND you chart your basal body temperature (taken before you get out of bed in the morning, after 3 hours uninterrupted sleep, and vaginally produces a more reliable result). When you get a clear shift in temperature you can be fairly certain (in a normal cycle) that you have O'd.
OPK's should be started about 5 days before you think you Ovulate and afternoon urine should be used. I usually test at between 6-8pm.
If you, like me, usually ovulate on cd17-18, I would test at 2pm & 8pm from cd15 onwards.
I'm rarely earlier than cd17 and know my cycle rather well, I therefore feel I can afford to test quite close to my O date.
The digi ones are imho the very best because they take the guess work out of OPK's completely. Either a yes/no, no interpretation needed.
Persona sticks are available in Boots and cost £9.99 per pack of 8 sticks. Ebay may sell them cheaper, as do a site called blushingbuyer.com, where I tend to buy about 4 boxes at a time. Delivery iirc is free and they work out a lot cheaper than Boots. Other chemists will likely sell them I would've thought.
The clear blue monitor and the persona are both small computerised devices which log info they receive from requested urine tests which you insert into the monitor on days where this is requested. They are essentially the same thing, except Persona is designed to be a contraceptive device and so will give green days (non-fertile), red days (likely to be fertile) and O signed days (Ovulation is likely in the next 12-24 hrs).
Essentially you would use Persona in the reverse and have sex on the red days & def on the O ones (it actually is a little egg sign).
The cb is the same but gives readings of low fertility, high fertility and peak fertility, but it all means the same thing.
They both have hefty price tags of around £70 new and will take about 3 cycles to recognise accurately your cycle.
You can buy these devices on ebay for a lot cheaper, sometimes MN'ers sell them. The data needs to be reset though before you use them.
Charting is free and will do the same thing, with the added bonus of enabling you to get rather more acquainted with your body than you maybe already are.
it can become a little obsessive though so be warned!
I have charted for 2 months after stopping bcp and have now stopped as I'm sure I O when I do. It's served it's purpose for me.
I'm maybe going to use the cb digi this month, haven't decided yet.
LP's of between 11-15 days are the most common lengths. Anything below 11 is less desirable because the egg may stand less of a chance of successful implantation if the lining is preparing to shed for menstruation.
IME an LP longer than 15 days is also unusual.
When charting 18 high temps (above coverline) is usually considered a sign of pregnancy.
I'll reiterate though OPK's charting etc is NOT for those with irregular cycles/pcos. persona is no use for those with long cycles either.
hth