We teach that sit means sit and stay in the puppy classes I help at now....
Obviously, as it's not gundog specific, we don't use a whistle command, but the method could easily be adapted to use whistle if you wanted to.
We use clicker to train... first lure into a sit position, click and treat... if dog stays in postion click and treat again and again for 5 repeats... (if your dog is used to clicker being a release, this might take some time to achieve...)... you then need to introduce a release command (we advise "free", though we taught "ok" for Honey when she was a pup!) - say the release command and encourage your dog to move.
Gradually space out the click/treats and gradually introduce handler movement and other distractions.
If you are just looking to sure up positions, look up Kikopup on You Tube, as they have a great video for really getting the sit, down and stand positions ingrained... it is their method which inspired the change of approach at our dog training school.
We also trained Honey sit-stay (as in sit means sit and stay) without using a clicker in our obedience classes....
We would put Honey in a sit, and continue to feed treats (always bringing the treat up to our chest before giving to the dog).... at first it was just keep feeding almost constantly... then introuce a little gap, then maybe a step away... always returning and giving a treat and calm praise. If at any point the dog moved, we just said "Blew it" and treats went away... no more sit-stay training for that session. Honey quickly learned that staying in position was rewarding, while moving wasn't.... over time you would put the pot of treats on the floor in front of the dog, while you walk away (up to out of sight), return and give the dog a treat from the pot, but this is after weeks of building up! For obedience we had a very specific release... we would return to the dog's right-handside, walk once around the dog and then pat the dog on their left shoulder saying "ok" (or "free"), this was so the dogs wouldn't get confused by judges walking around them in a competition situation.