Export professionals say they should concentrate on expanding into new territories and international markets in order to increase export sales in 1998.

Based on responses from hundreds of export and international sales managers across the United States, this ''first commandment of exporting'' should be framed and hung on your wall, according to this month's Report on Managing Export from the Institute of Management and Administration Inc.In the institute's survey, export managers said the five most crucial strategies implemented in 1997 to improve sales and marketing aspects of export operations were:

1. Selling in new countries/territories - 71 percent.

2. Developed a new or more effective international sales/marketing strategy - 69 percent

3. Developed new or stronger relationships with foreign sales representatives - 66 percent.

4. Asked staff to take on more responsibility - 43 percent.

5. Changed international pricing - 36 percent.

While the survey results are remarkably consistent - regardless of whether data are broken down by company size, percentage of product exported, or title of respondent - it is clearly the smaller firms that are most aggressively pushing into new markets, the report said.

For example, selling in new territories was the top tactic at firms with fewer than 250 employees (78 percent) and those with 251 to 499 (74 percent), while this strategy fell to second place at exporters employing 500 to 1,000 workers, and to third at those with more than 1,000 employees.

The data also show that the smallest firms are most aggressive when it comes to changing international pricing, while firms exporting the smallest percentage of product are most likely to feel the pressure to sell to international customers on open account.

At the largest firms (more than 1,000 employees), strengthening relationships with foreign sales representatives is the top strategy. Similarly, firms exporting more than 50 percent of product singled out developing their international networks of sales reps as the most successful single strategy for increasing sales.