In her article on beginner poker strategy, Catrina Rudd recommends taking it slowly instead of assuming you can handle a dozen open tables like the pros. This was the most helpful advice I found in her article.

Focus is a key that many people attempt to avoid by playing multiple tables. This does not just apply to people like Gus Hansen who may play as many as 10 or more tables at once, but also to the guy in his hotel room that thinks two tables would be slow enough to maintain an edge and double their money. The reality is that by playing more than one game at a time you are more likely to create reason to tilt. This will give you one opponent, one table, one hand to focus on. This facilitates creating opportunity to bring back the pot time and time again, instead of paying out the blind time and time again with no reprieve.

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