Insert a row with the ID you want MySQL to start at minus one. Say you want to start at 512, so you insert a row with ID 511. Then delete that row. That's it. Next row will be 512, then 513, .. etc.
Example:
Code:
mysql> CREATE TABLE mytest (
-> testid integer not null auto_increment primary key );
Query OK, 0 rows affected (0.00 sec)
mysql> INSERT INTO mytest (testid) VALUES(511);
Query OK, 1 row affected (0.01 sec)
mysql> DELETE FROM mytest WHERE testid = 511;
Query OK, 1 row affected (0.01 sec)
mysql> INSERT INTO mytest VALUES();
Query OK, 1 row affected (0.01 sec)
mysql> SELECT testid FROM mytest;
+--------+
| testid |
+--------+
| 512 |
+--------+
1 row in set (0.00 sec)