You want to be able to feel a work out with more resistance but you also want to be able to spin at 90 rpms when you are in a rest sequence. For instance, you should do some planned workouts on a trainer rather than just spin at the same cadence and resistance all the time. This will make you better. There are many variations, but here is a sample:

Begin with your resistance low enough on your tire so that you can spin at at least 90 rpm fairly easily. Do this for 5-10 minutes (10 minutes preferably) to warm up. Then do increasing intervals like this 1 minute on. This means click up a couple of gears and keep your leg speed at 90 rpm for 1 minute. click down to your original gear and rest for 1 minute; click up two gears for 2 minutes; then click back down for 2 minutes, 3 minutes on 3 minutes off, 4 minutes on; 4 minutes off. You have now been on your bike for 23 minutes. If you are only riding for approx 30 minutes, warm down at your easy gear for 10 minutes and you are done. If you want to ride longer, do a repeat of the above. always warm down at the end of your workout.

You get your resistance by changing your gears to a harder gear.

Another workout like the one above, but very similar is to do the same type of 1 minute on and 1 minute off, 2 minute on 2 minutes off; etc. but instead of trying to keep your leg speed at 90 (which means your resistance is not that hard), you change gears to a harder gear and practice climbing, i.e. your rpms will be around 60 rpms. You must be able to keep 60 rpms up for the entire length of the interval, i.e. 1, 2, 3 and 4 minutes. At first you may be able to do a higher gear for 1 minute but will have to do maybe a gear lower (easier) for the 3 and 4 minute intervals.

Hope this helps