Gamasutra posted an article a while ago about the art of Braid. It goes into quite a bit of detail.
In general, you can avoid having such an obviously tiled look through clever use of "variation" tiles and the addition of extra detail.
Let's say for example you have five ground tiles in a row. You might start off simply having five identical tiles. That's fine for testing purposes, and let's you get a good idea of the layout, but might get a little boring if you keep doing it. One solution is to add variation to your tile-set; you might for example create two more ground tiles, just by editing the original one to have some small variations.
By mixing these up when creating your ground you already have a less obviously tiled look, but a clever observer will still spot the tiling over larger areas.
You can take this a step further (or use this as an alternative technique if you don't want to create additional tiles) by adding some small details to provide additional variation. If you ever used the campaign editor from Star Craft, think similar to adding "doodads". As an example, you might take your line of five ground tiles, and then add a flower on top of one and a clump of grass sprouting from another. If you apply this technique cleverly, the "tiled" look becomes a lot less obvious.
You might also consider the use of additional layers (with or without parallax scrolling) if you aren't already using more than one.
Hope that helps!