ds has been seeing a speech therapist since he was 3½ yrs (he's 4½ now). To start off with he just wasn't talking much, and mainly what he was saying couldn't be understood, except by me and his key worker at nursery. We haven't looked back since. Once he started talking he wouldn't stop but he was having the problems you're all describing.
Basically, whenever we worked on his pronunciation we over exagerated the sounds that he was 'missing out/off'.
For example;
Snake was sssssssssssnake.
'b' would be used instead of 'f', to get round this, where ever there was the 'f' sound, we got him to rest his top teeth on his bottom lip and blow/flick his lip out.
The ST took things very gradually so that he didn't realise that he was having to work. She had games made up for him to play. We carried on this work/playing at home and it's really paid off. It only takes up 5 or 10 minutes.
If you are at all concerned contact your HV and ask for a referral.
After 9 mths of assessments and working with ds at home, only his teachers and ST could tell that there was a problem. I made a point of telling his class teacher about his speech, even though I'd already put it on the various forms I had to fill in for him starting school. She was really good about it and explained that there were a couple of other children in his year that were seeing a ST, and that they would get those children together occasionally to work on their speech. He's never come home from school saying that anyone has been mean to him about the way he talks sometimes.
BTW ds still has check ups with the ST, but these are now only every 5 or 6 months.
Apologies for waffling.