Statistics
Many sites (for example Paradise Poker and PokerStars) currently offer their users a set of statistics on their play. Whilst these are nothing like as comprehensive as those provided by a dedicated online poker tracking program such as Poker Tracker, they can still provide you with some useful information. For example, Paradise Poker provides a pop-up box which logs your statistics for each session and displays the following information in percentages:

Games won - This is not really a particularly useful statistic, since the purpose of playing poker is not to win as many hands as possible but to win money! Many players with a high ‘games won’ percentage will lose money overall, but will win a high number of hands purely due to the fact that they contest more pots to the river than they should. Furthermore, when you win a split pot it is counted in the statistics as a full win, which slightly inflates the games won percentage.

Showdowns won - This statistic provides a more practical indication of your play, since it reveals how often you display the winning hand at the river. If your long-run ’showdowns won’ percentage is considerably below 45% (for full 10-player hold’em ring games) then you may be seeing too many hands through to the river and may wish to re-evaluate your play accordingly. For tight players a long-run ’showdowns won’ percentage of 55%-60% should be perfectly achievable. However, the ’showdowns won’ figure should be used with some caution - if you would have folded to a bet on the end but it was checked down instead, then this goes down as a showdown lost.

Flops seen - This is the most useful statistic. There are no hard and fast rules on what percentage of flops you should be seeing, since in any single session your percentage will be largely dependent on how aggressively your opponents are playing. Indeed, a handful of extra free or cheap plays from the blinds can totally distort your overall percentage. The best way to gauge how loosely you are playing at any point in time, is to compare your own ‘flops seen’ percentage with that of the table as a whole - if you are above average then you may possibly be seeing too many flops.
As for the long run, if your ‘flops seen’ percentage (for full hold’em 10-player ring games) is consistently higher than 30%, then you may wish to consider whether you are playing too loosely - many experts would argue that it should be as low as 18-25% in the long run. (Incidentally, PokerStars breaks the ‘flops seen’ figure down into ’small blind’, ‘big blind’ and ‘other positions’, which is quite informative if you wish to analyse whether you are contesting too few or too many hands from the blinds.)

Win % if flop seen (or fourth street for stud) - This statistic is very closely correlated to the preceding one. A long-run figure of less than 20% may indicate that you are perhaps seeing too many flops with sub-standard hands.

Your actions - These percentages should give you an indication of how passively or aggressively you are playing, and will vary greatly from player to player depending on individual styles of play. As a rough guide, if your call percentage is significantly larger than the sum of your bet, raise and reraise percentages combined, then you may wish to consider whether you are playing too passively.

Where you fold - Again, these figures will depend greatly on your style of play, but they may provide you with an indication of whether you are folding too much or too little.

Naturally, these statistics only become meaningful with a representative sample size. With only 50 or fewer hands they offer only a very brief snapshot of play, and should not be considered at all reliable, but as you play more and more hands they will provide a much more useful indication of your play. Paradise Poker does offer the facility to run on these statistics from one session to the next (rather than resetting them from zero at the beginning of each new session), and by the time you have accumulated statistics on 1,000 or more games, they should offer quite a good reflection of your playing style.