If you have an auto_increment field on the table you can remove entries that are equal to 100 less than the new id. This could be performed inside a trigger using the last_insert_id() function which gives you the latest auto increment identifier.
This is something you have to decide. One way you are basically guaranteeing to have only 100 rows in the table at any single time, the other your table may grow as entries arrive.
One that that has not been mentioned previously in the posts concerns any indexes that you may have on the table where you are going to store the information. Inserts and deletes causes the indexes to become inefficient over time. You should also consider rebuilding these indexes to keep them optimized for access.