When you say you're "missed out intenally" Alps, what do you mean? Are you being put forward but fluffing an internal interview, or not just being put forward at all?
If the latter, you need to embark on some profile raising. I did this recently after being passed over for promotion on the say so of someone who didn't know my work at all. I realised I needed to make a bit of noise. Now, I should have done this by going to some of the friendly senior people I work with and asking what I needed to do and improve for next time. I actually got drunk with one of them and ranted and he did that for me, but I wouldn't recommend it as an approach. 
The end result was I had some mentors at work who could advise me and dump stuff on me to get me the experience and, more importantly, the exposure and support I needed. Which meant next time I didn't get passed over as I had people to speak to my case. You have to play the politics internally, doesn't matter how good you are if you don't have someone who is in the room when the decision is made willing to speak up for you.
On interviews/presentations. Practice is the thing. I study for interviews, whether internal or external. Internal I actually study harder as you really can't bullshit your way through them. If you have someone who can mock interview you, get some pretend interview practice in. If you have to do a presentation, practice it in the shower, in front of your baby, the horse, whoever happens to be around. I was also advised on a course once to do it in front of a mirror. This does make you feel like a prat, but it gives you an idea of what you actually look like presenting and can be very helpful. Made me realise I could be an arm waving loon so I try to tone that down now.
My worst problem is waffling - I get through this by trying to anticipate what the key questions will be and having some examples of this. So, for example, in my last interview I was changing field. It was obvious I would get questions along the lines of "and what makes you think you can do X when you've only done Y before". I had my answers down, I tested them out on friends, and I avoided waffling through my answer.
Give examples, if someone asks "how would you herd cats" don't say "well I'd try and lure them with chicken". Say "I'd try and lure them with chicken. This is something I did in my last job as Champion Cat Herder and found it worked very well. I bought some free range chicken, poached it in cream, and dangled it on fishing line in front of the cats. I managed to herd 32 cats into a small pen and increased our cat herding quota by 12%."
There are also some tricks you can do. My favourite is at the "do you have any questions" stage. I like to ask something along the lines of "if I'm successful in this job what would my key challenge in the first 3 months be" or "what would I be expected to achieve in my first 3 months/what would my first 3 months look like". This makes the interviewer picture you in the role and helps them see how amazing you would be. Plus you can then, if needed, drop in some more examples of how brilliant you'd be at those things.
My final bit of advice, is there any way you could do some interviewing yourself on the other side of the panel? It's a brilliant way for picking up tips and seeing what you do wrong yourself.