It's not what you say that matters, it's how you say it. The pilot landing at the local flying club in the R22 could be a multi-thousand hour ATP out on a jolly, whilst the person calling overhead in the S76 could be a low hour PPL simply operating the radio. All other things being equal, the person in the tower will form an impression of your ability based on the only evidence he has - that is how you sound. You will appear professional if you sound confident and keep it short. Who you are, where you are and what you want will often suffice.
The problem with answering your question is that every pilot, professional or overwise, will have differences in their mannerisms and exact phraseology. You can't work to a script. Having said that, if you comply exactly with CAP 413, you will sound like a TYRO. I don't know of any experienced pilot that would ask for a radio check on two frequencies on each radio; if you call them and you hear them reply, it's working! Asking an A/G operator for a clearance is also a give away.
The simple way to sound experienced? Get experience! Professional pilots have the same RT licence that you have.
In short - keep it short, speak with confidence, don't be concerned about using exact phraseology - you'll be surprised at what you can get away with!
