It's really important that you take your temp when you first wake up and before you do anything, including kissing your other half. If you take it at the same time each day but this is not when you wake up then it will be inaccurate as so many other things will have effected your temp. You also need to take your waking temp at the same time every day as you get hotter as the morning progresses even if you are still asleep. It's a pain I know as we all like to sleep in at weekends or days off. I was temping for about a year and used to set my alarm to go off at weekends to wake me up at the time I'd get up to go to work, take my temp and then go back to sleep. It is a real bug to bare but it does create a much more accurate reading. Also I ended up temping vaginally as your temps are also affected by if you have been asleep with your mouth open. The best reason for doing it vaginally is that hubby doesn't notice as the beeping is under the duvet and he can't hear it. In fact I'm not sure my husband even realise I was doing it from down there.
Hope this answers your question.
Good luck,
Becs