1. Try and get in a position where the wind is in your face as best as possible (I've had plenty of the come from the wrong direction and stand next to me) so don't assume that they won't come from behind you just because the wind is in your face.
2. Try and find structure where they are likely to be - ie blackberry patches as great places for them to hide and feed etc at diff times of the year (summer they love fruit and what do blackberry bushes have plenty of in summer? Blackberries of course) foxes have a sweat tooth and are often found where fruit trees are. Rocky hillsides are often great places for foxes to sun themselves on cold wiinter days where the sun hits the mountains early and warms the rocks so worth whistling where those sorts of structures are too. Foxes also love scrubby country - anywhere where they can hide and feel safe (shot plenty laying in tussocks too where they were bedded up having a arvo knap whilst keeping warm and concealed.
3. I usually call for about 20-30 sec then give it a rest for a few mins, whist waiting I'm scanning all around me to see if anything is coming in from all directions (make sure you sit in front of a bush or tree etc to break your outline up and to reduce the movements you make down where they aren't easy to detect. I use all manner of diff types of callers and whistles as often one won't work but something else will get them coming in.
4. I usually stay at a stand for about 15-20mins then move 300-400 or so along until I find somewhere where they might show up from again. I like creeklines where they branch off in 2 or 3 diff directions, foxes love creeklines for their highways and concealing their movements when responding to whistles.
5. It's not unusual for the smart ones to circle around type thing trying to test the wind for any unusual smells that might warn them that something isn't right so just keep scanning around 360deg type thing (as best you can) cos they are predictable but unpredictable in what they will or won't do. I usually have a CF rifle and Shotgun with me when whistling to help with those times when foxes want to just appear out of nowhere and just stand next to you (handy to have a 12g in those situations for the close action run offs) and I definitely prefer a CF over a Rimfire as it gives you more flexibility for the ones who want to hang out far and refuse to come in no matter what whistle or caller you use (why limit yourself with a Rimfire)
6. Mating season (seems to start around April-May you will often get pairs showing up so be ready) and latter on in the season they are less responsive to the whistle as they are more interested in mating. The cubs usually start coming out around the beginning of summer and are easy to shoot and are completely stupid (had them come right up to me and just stand there wondering what the hell I am (had two do exactly that a few years back when the wife and I were having a BBQ, I thought I'd have a whistle as it looked like a good spot and it sure was.
7. Listen for the birds, often when a fox is coming in you will hear the birds sounding the alarm to warn other birds that danger is near and in that case pay close attention that general direction to see if you can spot the danger.
That's all I can think of for now - good luck and get stuck into em.