There are two ways you can develop a character: by what makes sense in the campaign, or by planning. This article takes the planning approach to character development, and assumes that a player wants the best character he/she can get. Trying to explain your progression in the framework of your DM’s campaign is your problem…
That said, I will discuss several options for making your character better.
Specialize: Sometimes, it’s great to have a character that excels at just a few things. Why? Because you can be better faster if you don’t spread yourself too thin, obviously! You become really, really good at one thing, and with a balanced party to cover your weak areas, your single talent becomes a huge asset.
Despite all the racial bonuses available for non-humans, I still think that a human character is the best choice for an ultra-specialist because of the extra feat and skill bonuses. Most important, these bonuses come at first level (+1 feat and +4 skill points). Why do I care about first level? Because many campaigns simply don’t run for very long, and the earlier you can get the best combination of feats, the longer you’ll enjoy those benefits.
Example: Spell-casters are highly improved by an extra feat, when you can take both Improved Initiative (to enable you to cast area-spells before the enemy closes with your party), and Spell Focus: Enchantment, which makes early Sleep spells SO much more effective. A human fighter can take Ambidexterity/2 Weapon Fighting and an Exotic Double weapon at first level, or perhaps Expertise/Spiked Chain/Improved Disarm. You can also get a fighter-archer with Point-Blank, Precise and Rapid Shot feats at first level. Okay, so it could be argued that an elf’s superior eyesight and Dex bonus are fantastic traits for an archer – I won’t argue. However, if you change your mind and wish to multi-class, a non-human severely restricts you. Which brings me to…
Multiclass: Though the practice of heavy multi-classing has been taken down a peg in D&D 3.5, you can still get a lot more benefit from many classes than with one or even two. Class-feats are handed out aplenty at low levels, and non-spell-casters will still get the benefit of increasing Hp and base attack increases despite class-hopping. If your DM doesn’t mind min-maxing this way, and you don’t have a specific attachment to the feel of a specific class, then I have no idea why you wouldn’t class-hop with non-spellcasters.
Example: I’m going to use a Rogue/Barbarian/Ranger. While this may seem a strange combination, it is ability-heavy, and can even be explained as a bandit-turned-wilderness-survivor. For this combination, you get: high skill points, sneak attack +1d6, rage, +10 move, dual wield, tracking and a favored enemy, plus your usual two feats for levels 1 and 3. You also get 4/2/0 saving throws by third level and an average of 17 Hp by 3rd level. Further progression with this same character: Rogue/Rogue/Fighter/Fighter. You get Evade, Uncanny Dodge I, 2d6 Sneak Attack, two more levels of high skill points (Rogue) and Hp (Fighter), and two fighter feats.
Spellcasters have a different reason to do some multi-classing: hit points and weapon proficiencies. They also benefit from skills and feats, but taking fighter before sorcerer suddenly gives you double the average sorcerer hp at 2nd level, at the cost of one spell/day! You will be delayed one level getting your next spell level, but you will be much more likely to survive until then. Does anyone else hate 4 hp, 1st level mage-types? Or hate avoiding the valuable “Improved Initiative” feat because you feel you have to take Toughness instead, just to survive?
Planning for Common cases or Worse Cases: While it’s nice to be able to clobber monsters on a regular basis with your Bastard Sword/Cleave/18 Str human fighter, it’s extremely annoying to have this character be captured, stripped of weapons and armor, and put in a dark, heavily-barred cell and somehow have to escape. A half-orc fighter isn’t much better, since they will have less good skills with which to get out of difficult, non-combat situations (including swimming, climbing, intimidation and the like), and they have less feats (but at least they aren’t afraid of the dark)! Rounded characters are especially important in campaigns with few players where a cleric has to be both a front-line warrior and healer, a rogue has to be the talker, trapper, rear-defender and scout, the fighter has to be good with a bow and sword and be a damage-absorber, and the mage somehow has to survive low levels that include melee combat for all party members.
A devious DM might introduce many situations where specialized talents become useless: you lack weapons, your targets are immune to your specialized spells, you are put in an unfriendly environment (such as water or darkness), or your opponent has ways to take away your advantages (disarm feat, touch attacks vs. armored fighters, flying vs. melee specialists, etc). With a devious DM or few players, a well-rounded fighter might want to forgo the weapon focus, or even the Power attack/Cleave combination that is so deadly, and opt for Improved Unarmed Strike and either Blind Fight, Improved Initiative, Combat Reflexes, or Expertise.
A second but important tip for fighters and clerics: instead of concentrating your best stats in the physical areas, consider putting a 14 in Intelligence. Doing so doubles your skill points gained for each new level! Skills can really get you out of trouble, especially when it’s something each character has to succeed at, or else bad things happen (consider Spot or Listen to avoid surprises, Swim to avoid drowning, Diplomacy so you too can talk once in awhile, Knowledge skills to get hints from the DM, and Move Silently so as not to screw it up for others).
Example: A cleric might take Lightning Reflexes. A sorcerer might forgo a super-high charisma and opt for Con instead, or a rogue could take a level of cleric (healing, hp). A fighter could take a level of ranger (skills, track, dual-wield), or a level of barbarian (rage, movement, skills).
Exceptions: Druids and Bards. These are extremely well-balanced classes, except perhaps for the Druid’s limited usefulness in cities, and the bard’s armor-vs-spellcasting dilemma. Druids get additional abilities almost every level, good hp, and spells. The only thing I might suggest is a level of rogue (if half-elf or human) before starting on the druid path because of city-adventures, but the druid gets a good number of skill points already, and sneak attack of +d6 and a few more skill points doesn’t balance the delay of higher-level spells in this case. You may be shouting: “But clerics are the same thing!” They are not. They get half the skill pts/level, are not restricted to certain weapons after multi-classing, and they do not get many new abilities per additional level. Bards are already multi-talented, and don’t need extra classes to help their development, unless you’re looking to swing your bard towards thiefdom (recommend up to three Rogue levels), or towards combat (add one or two Fighter levels).