"I've gone back and tried to see the main differences and I'm trying to get back to the way I was then," Australia's quick bowler said on Tuesday.
"The last couple of days up here (at a training camp in Brisbane) have gone a long way towards sorting out that action," McGrath added.
He and Test opening batsman Justin Langer will play for the Northern Territory Chairman's XI in a four-day match against Sri Lanka starting on Thursday in Darwin. The first Test starts there on July 1.
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The 34-year-old, whose last Test was against Bangladesh in July last year, said he had been motivated by a comment from chairman of selectors Trevor Hohns that he needed to show his worth to the side.
"I guess those comments do spur you on a bit but I've been happy with the way I've progressed," McGrath said.
"People shouldn't expect that I could come back after nine months off and slot back into my best form."
"The big thing people have been saying about me is that I need to bowl quicker but I've never been an express bowler."
McGrath, who has taken 430 wickets from 95 Tests, added: "Talking to batsmen around the world, they've always said it's been that extra bounce that I get that troubles them the most."
"I just want to get that bounce and nip back. If I get that back no-one will be complaining any more."
