I've mentioned using Detergent 8 for removing flux in a past post but there are some other products that I've used/experimented with. I've found that while Detergent 8 overall does a good job, some particular burnt-on flux can remain. I've found a product that will get just about anything off: Goof Off Pro Strength remover.

WARNING: I looked up the MSDS/SDS for it and it's acetone based, so soaking a PCB in it would not be a good idea in most cases as it will attack certain types of plastics.  The best way I've found to use the product is putting a few drops on a swap and wipe the PCB with the swab.  This allows one to easily control application and thus easily avoid contact with any plastic parts mounted on the PCB.  Also allows a little amount to go a long way.  Another nice thing is that it doesn't appear to leave any annoying residue behind.

Another product that I've been trying out with success is Ammonia.  It smells bad (use in a well ventilated area), but it's extremely cheap and seems to do a reasonably good job via putting some on a paper towel and wiping the board. 

Again, one has to be careful with using products that aren't labeled to remove flux off of PCBs.  The safest options are Isopropyl alcohol or a product specifically designed for flux removal such as Detergent 8, but I do find Isopropyl alcohol not as effective as I would like it to be (have used 90ish percent but haven't gotten a hold on 99%) and while Detergent 8 is effective overall, it's rather expensive.  It's nice to have cheaper and effective alternative options.