What is a good mining pool/payout model for infrequent mining? I usually mine only, when I do stuff using my PC so it is on anyway. So far I used Slushs Pool but they have a score based reward system, which seem to punish casual mining. If a long round ends >15 minutes after leaving the pool (due to shutdown the system) you'll probably get nothing at all. What are better payout models for the casual miner and what pools can be used?
There are multiple approaches to pooled mining, each with its own benefits and detriments. Assuming you are a dedicated miner who does not pool hop then the scenario you describe should be an "edge case" that should rarely if ever occur. That said, common payout schemes include:


PPS - Pay Per Share. Each submitted share is worth certain amount of BTC. Since finding a block requires  shares on average, a PPS method with 0% fee would be 50 BTC divided by . It is risky for pool operators, hence the fee is highest.
SMPPS - Shared Maximum Pay Per Share. Like Pay Per Share, but never pays more than the pool earns.
ESMPPS - Equalized Shared Maximum Pay Per Share. Like SMPPS, but equalizes payments fairly among all those who are owed.
CPPSRB - Capped Pay Per Share with Recent Backpay.
Prop. - Proportional. When block is found, the reward is distributed among all workers proportionally to how much shares each of them has found.
PPLNS - Pay Per Last N Shares. Similar to proportional, but instead of looking at the number of shares in the round, instead looks at the last N shares, regardless of round boundaries.
Score - Score based system: a proportional reward, but weighed by time submitted. Each submitted share is worth more in the function of time t since start of current round. For each share score is updated by: score += exp(t/C). This makes later shares worth much more than earlier shares, thus the miner's score quickly diminishes when they stop mining on the pool. Rewards are calculated proportionally to scores (and not to shares). (at slush's pool C=300 seconds, and every hour scores are normalized)


The important factor is that you avoid Proportional payout systems as they are vulnerable to pool-hopping so you will probably lose some portion of your revenues to hoppers on those pools. For your concerns, SMPPS, ESMPPS or PPLNS would be good choices, but there are other factors to consider and it is ultimately up to you to choose a payout scheme and pool that best fits your needs.