Hi Duce,
After a lot of practice I've actually found it to be easier to do a lot to techniques when your opponent is tense; they're balance is often much easier to break. But this takes some time to develop.
Find someone who will safetly train with you. Let them grab you really really hard and tense. Then you relax as much as possible. No very slowly feel your way through the technique. If you feel yourself tensing up or not taking your opponent's balance, stop, relax, and feel your way through again. Concentration on keeping extension, and all the other basics.
The other thing to remember if that if someone's really tense, you will have to pick you technique based on their posture, momentum, etc. For example if they are learning really far forward, instead of doing an iriminage and trying to lift them all the way back up again, keep them going forward with a kaitenage.
My last piece of input is this: You can learn how to throw a decent kick or punch in a couple of days (Note that I didn't say master). It takes much more training to learn to evade, keep extension, focusing on balance, and proper maai. However, all of these things are necessary to properly execute any aikido technique. Thus a karatedoka with 1 year of training will look (and probably is) much more effective than an aikidoka with 1 year of training. I think people look at this and say that aikido is ineffective when it seems like much more of learning curve difference. Let the same two karatedoka and aikidoka meet again in another 5 years and you may have a different story.
Good luck and keep training.