which fields should be indexed to speed up a query

here is the query but is way to slow

Code:

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SELECT dbfPATIENT.PNAME AS PatientName, dbfFANAL.RESULT, dbfEXAM.EXNAME, dbfEXAM.RCOLS, dbfEXAM.RROWS, dbfEXAM.RPIC, dbfEXAM.ANSPIC, dbfDOCTOR.DOCNAME, dbfDOCTOR.DOCTOR, dbfFANAL.FANAL AS FANALID
FROM dbfDOCTOR INNER JOIN (((dbfPATIENT INNER JOIN dbfIDFILE ON dbfPATIENT.PATIENT = dbfIDFILE.PATIENT) INNER JOIN dbfFANAL ON dbfIDFILE.ID = dbfFANAL.ID) INNER JOIN dbfEXAM ON dbfFANAL.EXAM = dbfEXAM.EXAM) ON dbfDOCTOR.DOCTOR = dbfIDFILE.DOCTOR
WHERE (((dbfFANAL.RESULT) Is Not Null) AND ((dbfDOCTOR.DOCTOR) In (340,18,194,446,171,66,99,24,28,34,61,83,141,175,33,78,127,177,156,112,244,238,18,240,312,51,102,107,136,533,506,178,508,487,475)) AND ((dbfFANAL.PRINTED)=0));

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index the fields that contain only unique data for that record. such as an ID field or something similar.

Also to the fields you are heavily ordering on (not in your SQL statement) and the fields you are heavily searching for, in your SQL statement:

dbfDOCTOR.DOCTOR

And on the fields with unique values in each table you are using in the query.