Well I haven't used them yet, but on Saturday I brewed up an IPA that I will either spice a part of it in the secondary or add a vodka infusion at bottling. I am slated to use fenugreek and coriander in two separate batches and perhaps blend them in a third, to compare to the base recipe with no spices. It is a modified recipe that is found in the book Radical Brewing.
Coriander is the more common spice to use. If you'd like to use that, source some indian coriander (pale, rugby/football shaped) rather than regular grocery store variety. This is more citrusy/floral rather than vegetal. As for an amount to use, 0.5 oz per 5 gallon (witbier) is probably a decent to low starting point, if used at the end of the boil. Of course, this might depend on how hoppy your IPA is. Cracked, slightly ground, these also can be added to secondary, provided you sanitize them in some way (boiling in water or other means). You can also steep coriander in vodka while your beer is fermenting, filter, and add it to taste to either secondary or your bottling bucket. This allows you more control over its contribution to the overal profile.
Each method may be different in terms of preserving flavor and/or aroma, and combining methods may add some layer of complexity.