I agree, knock backs happen, what I don't agree with is that they are necessary.
Getting knock back happens, but you wont become confident and relaxed around women by having a huge catalogue of knock back in your arsenal you can call on. In fact I'd say it would dent any existing confidence in such situations.
I know it sounds like some self help book advice, but the reason you are relaxed and feel good around other is when you not thinking things about yourself. Like omg are they looking at my baggy clothes and weedy arms, or fat gut and balding head.
I know this from experience really, it might not require muscles or new clothes, but whatever the underlying problem is, it has to be fixed first before you try, and then you might not get knocked back.
If you can walk into any situation focused on the job ahead, without having half your brain worrying, it will show in your body language and conversation, smile etc.