Do any of you girls yawn much in front of clients?  I dunno what it is but i literally cannot stop myself. 
Recently done an overnight and it was a night mere as the guy was actually so interesting but i found myself yawning every 5 mins and the guy got quite offended.  I think its maybe a sign i shouldnt be there lol. 

My top tips for disguising the great yawn:

yawn whilst taking a fake sip of water
yawn behind the yr elbow whilst fake scratching ur shoulder blade
yawn whilst pretending something the other direction caught ur attention

sorry this is so random just wanted to share this

I do not do overnights because of this.  If someone who I am very near to yawns I then have a yawning fit for about an hour where no matter what I do I cannot stop.  It has happened a few times during normal bookings but the guys thankfully find it hilarious when I explain that they started me off and I even have one who will try his hardest to yawn to see if he can get me at it. 
I wouldnt worry.  Yawning is not a sign of boredom or even tiredness is can be a sign of being very relaxed or even of sexual arousal (sources: Alexander, The Stress of Life)
Explain this to them and they should be just fine about it

As Dani says yawning isn't necessarily because you're bored or tired, it's how our bodies get rid of excess Co2, being inactive and relaxed is often the cause so go ahead and yawn!  I find it more of an insult if I see somebody try and hide it personally! 

I have learnt how to turn a yawn into a sigh of pleasure!

Quite often a few really deep gulps of fresh air gets rid of the yawns, like Diana said, excess CO2 is often the cause and can be exacerbated by a stuffy room with inadequate ventilation (think two bodies heavy breathing for hours at a time), if actual tiredness isn't the cause. Ask to open a window or stand at the back door for a few minutes (assuming you're not in the middle of nakedness!).

I don't suppose telling him he's a sex machine and he's tired you out is really acceptable way of getting to get to sleep a little earlier!

Yawning is a sign of both built-up CO2 and lack of oxygen, so if you've been "going at it" for a while, or are in a stuffy hotel/apartment, open a window, take a minute to get your breath etc. Or as daft as this sounds, stretch your legs out. (like the in-flight exercises for long-haul flights to prevent DVT). I'm doing this at uni and it's all about blood oxygen levels and blood flow to the brain. It's not always about being tired, like OP said, being relaxed or aroused can also make you yawn.

I definitley agree, hiding it and/or not apologising is awful! I also suffer from uncontrolable yawning fits, usually around mid-morning, so any 10am clients get the full brunt of it! LOL.

I'm yawning reading this.  It's weird how the body and mind sometimes work. lol