Some general stuff ahead:
To avoid performance problems, you have to - among other things! - maintain the indexes periodically. Maintenance in this case means, to restore the index-pages by re-adjusting the defined FillFactor. The FillFactors have to be optimized to have the best balance between cache-usage and Read/Write-Performance:
Low Fillfactor = Big Index = Poor Read/Good Write (less PageSplitts)
High FillFactor = Small Index = Good Read/Poor Write (lot PageSplitts)
So, the FF basically depends on how many read/write transactions you have on a table. If your table is causing trouble, you could try to adjust the FF, a too high fragmentation could indicate the the FF is too high (= many PageSplitts), so you could try to set a lower FF. But take care! If the FF is set too low, performance will also decrease! Usually, you have a standard FF of 90%, so maybe you could try 85% and see what happens … (and: never go below 70%!)
Regarding index maintenance:
To rebuild the FF, you basically have these options:
DROP the indexes and re CREATE them from the scratch: Best and cleanest solution, but time-consuming and no parallel user-activity is possible during re-creation
DBCC DBREINDEX: allmost the same as DROP/CREATE
DBCC INDEXDEFRAG: Not as clean as the two options above, but could be performed with parallel user activity
Aditionally, you also need to maintain the statistics - don’t forget!
My personal opinion is, that the “standard” maintenance tools of NAVISION (table optimizer) or SQL Server (Maintenance Plan) are insufficient. To set up a more precise maintenance, you have to have some SQL Server skills as it is not that easy … In our subsidaries we have implemented SQLSunrise (www.sqlsunrise.com) to get the most efficient maintenance easily …
Finally, one word to the NAVISION Table Optimizer:
This thing is resetting all index setting to the IMHO very insufficient NAV standard, all adjustments made from “outside” are gone … I really hate this thing 
Hope this helps a little!