Never yell a command at dog; a trained dog only needs a firm voice, and an untrained dog will bark back at you.
‘Down’ is the wrong command; it can mean ‘lie down’, ‘get off the couch’, or ‘let’s dance’.
Body language is game playing; you are a human, use words
Tell the dog to ‘Sit’. Firmly. (This works with dogs of all languages, by the way; when a dog hears an unfamiliar command, 9 times out 10 (s)he will sit.)
Then give it a scratch behind the ears, and a tennis ball.
Hey, (s)he likes you; affection and good humour is too rare to dismiss as an annoyance.
That being said, how can I keep you cat off my lap?