When should students look for summer jobs? Now!

Lisa Campney, a Dartmouth junior, already has a $1,000 a month job lined up next summer in public affairs at the Mobil Oil Corporation. Ashok Pandey, a student from Nepal working on his MBA at Harvard, last week had interviews for summer jobs with two US firms that have offices in Asia.

Keith Jenkins, a freshman at Princeton, has just finished his resume and will soon be contacting New York City firms for summer jobs in engineering.

These college students and hundreds like them took the advice of their college career offices and started jumping the hurdles for summer jobs soon after fall classes began.

"Students who want paid summer jobs involving more than clerical-type duties should waste no time starting their job hunt now," says Richard Howard, an adviser at Harvard's office of career services. To succeed when competition is tight, Mr. Howard says, students should look for a summer job as though they are looking for a permanent one. "Write a resume, research the job market, get in touch with everyone you know."

Some employers may not know if they'll be hiring summer workers until early spring, but students should use their winter vacations to go in and get to know the company they are interested in anyway, he says. Britta McNemar, director of career and employment services at Dartmouth College in Hanover, N. H., says, "Just a few years ago deadlines for summer job applications were April 1, now they're being pushed up to early January and February. Some summer slots are already being filled."

The trend of businesses to groom liberal arts graduates in summer jobs and bring them back after graduation is continuing, career advisers say.

Lauren Stolper, director of career services at Pomona College in Claremont, Calif., says, "We're finding more and more companies receptive to bringing liberal arts students into business because the MBA graduate is twice as expensive. Many firms have summer programs which offer professional experience -- not just jobs on an assembly line -- but mini-management experience."

Nathan Low, a Harvard junior majoring in history and literature, last year sent letters to 12 company presidents and within a week had 10 responses. Six firms offered him summer jobs; four asked that he call them for a job after graduation.

When asked why he had such a high rate of response, Low explained, "I only wrote to presidents. . . . I didn't bother with personnel offices."

He accepted an offer from International Rectifier Corporation, which manufactures semiconductors in Los Angeles, and spent his summer programming a budget manual for department managers and selling $250,000 of surplus machinery.

Ms. Stolper says students seeking summer jobs must define their priorities, deciding if they want to earn money, get career experience, or both.

Students who want their summer vacation to be flexible, yet want to gain valuable work experience, should consider applying to work at temporary agencies that specialize in such career areas as publishing, market research, or computers, the experts say.

Directories which offer leads on summer jobs include:

* Student Guide to Summer Jobs in Business, published by the Intern Research School of Journalism, University of Colorado, Boulder, Colo., 80309.

* National Directory of Summer Interships for Undergraduate College Students, published by Bryn Mawr and Haverford colleges, NDSI career planning office, Haverford College, Haverford, Pa., 19041.

* Storming Washington: An Intern's Guide to National Government, published by the American Political Science Association, 1527 New Hampshire Ave. Northwest, Washington, D.C., 20036.

* Summer Jobs in Britain for 1980, published by the London firm Vacation Work , distributed in the US by Writer's Digest Books, 9933 Alliance Road, Cincinnati , Ohio, 45242.

* The Directory of Summer Opportunities, AIC Inc., Box 154 EBS Andover, Mass. , 01810.