If you have any ad blocker installed and it blocks a ClixSense ads, you cannot complete clicking it. So, you may have to disable your ad blocker temporarily.
There are ads of different values. Some are of $0.001 while some are of $0.01. Sometimes high value ads are not immediately shown on the ad page. When you click any normal ad first, the high valued ones show up.
There is no direct referral limit, you can have as much as you can even with a standard membership. So, if you even are not interested in upgrading, you can still have a large number of referrals and increase earning with them. And, of course, upgrading will bring you higher click commissions from your referrals.
ClixSense also pays standard members $0.50 after his referral earns a total of $10 and premium members $1 after referral earns a total of $5. Also, you will get $2.00 per upgraded direct referral.
I have already talked about these in the general tips page. ClixSense has more offers and tasks than other sites and you can try these to increase your earning. The availibility of these depend on geographic location. For example, a user from USA will certainly see more and high valued offers or tasks than a user from Bangladesh. Some people earn a huge amount monthly by completing those and I cannot give further detailed tips because I am not one of them. :).
ClixSense has a low upgrade fee of only $17.00 per year. But you have collect some direct referrals first to benefit from the premium membership. Check out their 'Premium account' page to learn more about upgrading.