1. Yes, it'll just work less.
Spells are only as strong as:
- how well you can focus on the specific goal during the working (Void, Focus Meditation)
- how deep you are in trance (Trance training)
- how much energy is raised (general experience improves this, as well as a healthy body, but pranayama and Yoga are direct in improving this)
- choice of affirmation, mantra, repetitions, patterns, etc. (all knowledge available here)
- external factors that offset or help, such as planetary positions, your own chart (can be tuned via Squares), other people wanting the same thing, what exactly you're trying to improve and how much karma there is in the opposite direction, etc.
2. For the vast majority of people, the main mistake being made is assuming that perfect void is needed to make any progress.
If you just try, daily, you will still make progress even if the session seems like it failed. Even a few seconds without thoughts, a minute focusing on a flame or a texture or sound, a little effort towards not getting lost in thoughts, it all compounds.
There's no way to "do it wrong" in the sense that you're not improving at all. Just keep trying and see it improve over time.