After being on the outside for years, leaders of some of the nation's largest conservative groups yesterday tried to prove that they know how to act on the inside.

It was a united front of support for President Reagan's economic program, but at the same time tried to make it clear that they value their independence and have a long social agenda to which the conservative president must pay heed.

"We're not in anybody's back pocket," Robert Heckman, executive director of Young Americans for Freedom, told the opening session of a four-day Conservative Political Action Conference expected to attract 1,000 people. "We stand for ideals and principals. We must retain our old goals as vanguard on the right and keeper of the faith."

The conference is the first time a large number of conservatives have gathered in Washington since Reagan's January inauguration, when many of its leaders were still handy with last November's victories.

"All that we have been trying to achieve for so many years has finally come to some kind of fruition," said Rep. Mickey Edwards (R-Okla.), chairman of the American Conservative Union. "What we have to make sure is that what happened last year is not a one-shot deal, but as I believe, the first step toward a conservative movement that is going to continue for the next 20 to 30 years."

Edwards and other spokesmen were firm in their support of administration policies to date. Richard Viguerie, the conservative direct-mail expert, said he is concerned about Reagan's failure to appoint more conservatives to federal jobs, but added, "I feel we should be doing everything we can to help the president get his economic program passed."

There will be time later, he said, to push for "social issues," involving such items as abortion, busing, school prayer, and sex on television.

Reagan and Vice President Bush are to address the group today.