As I work on adding new stuff to existing database there is good chance that indexes will get fragmented. On quite a few occasions I got them in 90% fragmentation range. And that affects performance.
SQL Server 2008 has two main ways to fight fragmentation. One is REORGANIZE command (e.g. "ALTER INDEX ALL ON MyTable REORGANIZE"). This will do simple shuffling of existing data. No new allocations will be done. Unfortunately this also means that, especially in case of multiple files, some things will stay unoptimized.
For worst-case-scenarios there is REBUILD (e.g. ALTER INDEX ALL ON MyTable REBUILD). This command creates new index instead of current one. This will probably cause some new allocations and thus it will take a little longer, but final result will be impeccable.
Microsoft has really nice page that explains both how to check fragmentation and what can be done to solve it. Rule of thumb there is that you should REBUILD once fragmentation is over 30% and REORGANIZE when fragmentation is lower. I view this as quite good rule if you have SQL Server Enterprise. However, if Enterprise is not your edition of choice, things change a little.
Since I mostly work with SQL Server Express, only question that I ask my self is whether database can be offline. Below Enterprise edition, REBUILD operation CAN NOT be done while other operations are pending. If database has to be accessible, only option is REORGANIZE.
I do not mind that much since REORGANIZE is usually sufficient. And, if changes are large enough that they would cause real fragmentation mess, they are also big enough to allow for offline REBUILD.
I find that Microsoft has right to limit some functionality (in this case online rebuilds) to certain editions of their software. However, I do hate when they don't find it necessary to document this "small" difference. On other had, if Microsoft did all documenting, I would have nothing to write about.
P.S. This represents my view as a programmer, database administrator might not approve this. :)