If after successful launch product, it started decline, what to do to increase sales/availability?

Managing Categories is both Science as well as an Art and not science / Art alone. It is Art, becoz you need to understand the product design /specification, consumer behaviour  and usage, which will help you to cater to customer requiremnet effectively. It is science, when you Analyse sales  and get insights about the historical sales trend. One has to manage the product lifecycle,either by reinvent the product or introduce new / upgraded range to keep the customer interest alive. A buyer's job is to maintain the balance between the two. How effectively the balance is maintained, reflects in the category performance....

Analyse the cause of decline

Re-position the product

Boosting on the marketing is required to increase the awareness among the target customers. The campaign requires to highlight the salient features of the product.

It is important to identify the life cycle stage of each product before the launch and how to move the product through each stage.
In some cases you might see the product accelerating through certain stages as market conditions change.
Mr.
Farrukh above is right in identifying some of the causes as in some cases life cycle stage could be very short, due to the category or the fierce competition.
The product might need more trade marketing support to push it through the channels.
Remember a marketer has a number of push and pull tools at hand.
The key is to identify what the problem is, and using the right push and pool tools to rectify them.
It could easily be an issue of price positioning especially if the product is not significantly differentiated or the brand does not have a significant loyal consumer base.
Then you would see the sales tapering out after the initial launch campaign or as soon as the promotion ends.
If there is an issue with the product itself, it needs to be panel tested to identify the right formulation or any quality problems before the reformulation process can begin.
If it is a new product/concept then it is a good idea to bundle the product with another hero product and do sampling activations to start trial generation as consumers are hesitant to spend on something they are not sure of or do not clearly understand.
Availabilty and visibility are key for the success of any package consumer product to generate shelf off take.

It is easy be successful but it is really hard to maintain this.
In the present scenario, when market competition is high, product age is short and several alternatives are available for end customers, the ideal option would be to discontinue the existing product and re-launch with little modifications and a new name.
But this would be suggested for the organizations that are dealing with the product with very short age.
Other options for to promote existing products could be Even Promotions, Target Marketing, Value Added/Bundle Offers.
These can be used by any kind of organization to promote the existing product.

1) Pricing might be the basic criteria2) BTL AND ATL Activities3) Product Positioning.